This is a phrase that was popularized by U.S. President Dwight D. Eisenhower during his farewell address. In it, he warned to beware the dangers of the "military-industrial complex". The initial drafts of this speech actually include the wording of "military-industrial-congressional complex," which is really more what he was talking about, but the congressional portion was removed to not upset politicians. This is an important point to make, as congress is an important player in the complex.

Anyway, the idea is that the way these three things interact creates huge conflicts of interest between them and it generates a feedback loop of corruption and wasteful spending. So you have politicians in congress that want to continue to be voted in - the easiest thing they can do to pad their resume is point to jobs that they've generated for their constituents. Since they're in the government, they can vote for bills that will award big contracts to companies that are based in their districts. Since the US military is so huge, most major contracts are defense related. This generates jobs for the citizens of that district, generates big money for the company that gets awarded the contract, and helps the politician get re-elected. So everyone wins, right? Well, not really. When this starts happening, there's a feedback loop that occurs that starts to inflate the prices of everything because everyone wants a piece of the pie - corporate lobbyists spend a ton of money on politicians, politicians want to spread out the jobs so it becomes difficult to effectively build things, and the military likes fancy expensive things. But the really big issue is that it promotes extremely wasteful spending; when it's in everyone's best interest to just fund a ton of military projects, things that get funded are not actually beneficial to the military capabilities of the country and instead it starts focusing on whatever benefits corporations and politicians the most, because the military stops actually being the main beneficiary of military spending. And once we have all this bloated military spending and equipment, suddenly using it in conflicts to justify its existence becomes in the best interest of politicians and corporations that benefit the most from it.

It describes the relationship between the military and arms industry where industry groups rather than foreign policy, dictates military policy.

It is a combination of a military "class" whose careers advance by having conflict, and the manufacturers and various others who profit from conflict. These groups will presumably combine to lead the people of a country in to wars that serve mainly to benefit those groups.

This, of course, lets the people of the country you are discussing off the hook for the actions of their government, which is by and large HOOOOOOOOOOORSE SHIT. People love wars, until they decide they were against it all along, which always happens after there is not the quick victory they thought would happen.

Also, FUN FACT- This was regarded as some kind of peacenick pronouncement against war when Eisenhower warned against it. This is not so, what Ike objected to was large scale, conventional wars. He was the man whose administration oversaw many of the coups of the cold war, and set policies that would see the 3rd world in flames for decades. He just did not want to use american soldiers and tanks to do it.

The military industrial complex is a reference to the entirety of the corporations and their industrial factories they created to make weapons of war and destruction and protective gear and deterrents or vehicles to drive and deliver those weapons and gear.

It refers to the buildings the work force and the people that profit, even, technically from selling weapons and protective gear related to fighting and war.

The idea that you have those at top of military and top of defense contractors working to keep their influence high… military demanding ever increasing budgets and new weapons, vehicles, etc. while the defense contractors making them lobby Congress to help the cause.

And you’ll often see high ranking military or elected officials go to work for defense contractors as lobbyists or other high paid positions when they want higher income, so their work while in government is often looking forward to personal enrichment by doing favors vs. actual best interests of military/country.

It’s also about the arms race. As you advance, so does your adversaries, so you have to keep advancing to maintain your advantage. You always need new, because the old gets destroyed or less and less effective over time. Once it becomes a large part of your economy- it’s vital to towns like any other big industry, and any cuts have have huge economic and political repercussions.

It’s the collection of business people who profit from selling goods and services to military. They have corporations, companies, and even small business that provide for the military. Companies that produce ships, airplanes, tanks, other military vehicles, all their parts, missiles, ammunition, dresses, shoes, rations, communication devices, toilets, tents, computers, software, etc.

There are also academicians who work for them. Journalists who fork for them. Politicians who works for them. Intelligence organization personnel, judges, prosecutors, lawyers, etc. all who work for them officially or clandestinely.

All these people comprise a social network like a “state”. They actively push for policies that would result in more military spending.