First, you're taking a bit of a gamble there. It might not taste bad, but that doesn't mean it isn't.

With that said, there are many factors that'll keep food from spoiling. Properly sealed food with all the air removed is one (and that might be the case with your salsa). The food itself being inhospitable to bacteria is another, and refrigiration is yet another factor.

Bacteria are usually what cause food to spoil, but they may require oxygen. The jars are pasteurized (sterilized) and sealed during production, this kills any existing bacteria and keeps others from getting in. Once you break the seal, the microbes floating in the air can get in it and cause it to spoil.

the fact it has been sealed for that long means no air not additional moisture got in. most food products are packaged in a way that it removes air/humidity and sterilized(often thru the process we call Pasteurization).

as long as the product remains sealed(before you 1st open it yourself+ the packaging is undamaged) you can generally be safe in the knowledge it will be good at least until their "best-by" date and that it hasnt been tampered with.

tho fair warning, depending on how it was stored, this doesnt mean it still "good", temperature extremes can alter the charactirstics of the product.

The canning process kills bacteria with heat (the jars are “cooked” in hot water) and then vacuum seals the food so there’s no oxygen to give life to any remaining bacteria to live on.

The little pop when you open a jar for the first time is you breaking the vacuum seal and letting fresh air in for the first time since the food was put in there.

Basically bacteria, fungus and oxidation are what make food go bad. Without these things, decay doesn't happen. In fact before certain bacteria and fugus existed on earth, vegetation would just pile up in massive amounts. If I am not mistaken, this is why we have oil and coal. Massive piles of vegetation built up, which allowed for those to slowly convert to fossil fuels, since they didn't get eaten by bacteria and fungus.

So basically the canning process involves a few things.

• Cooking
• Preservatives
• Air tight seal.

You cook the food until all contaminants have died, then you seal while it us hot, which removes most of the oxygen. Since the hot air is thinner, when it cools it creates a vaccum seal with minimal oxygen, and prevents anything else from entering. Canned things can have vinegar, salt, citric acid, or other preservatives also, those also prevent any bacteria growth.

You now have a sterile environment that doesn't allow for decay, at least for a very long time. It isn't 100% perfect though.

The reason it doesn't go bad is the sterilization/canning process, not the glass container itself. The jar of salsa was sealed and heated to the point any microorganisms inside the sealed jar were killed and the fact it is sealed means no new microorganisms can get in preventing spoiling. Properly canned food can theoretically be safe to eat indefinitely as long as the seal isn't broken and it's kept somewhat cool though the texture/taste can definitely degrade.

Glass jars are just a convenient container for canning food in, but are not doing anything special to prevent spoiling.

It's not so much the container it's kept in as the conditions it's kept it. Refrigerator temperatures slow the growth of bacteria and fungi, and the tight-fitting lid prevents air from oxidizing the food. And preservatives added by the manufacturer are likely to do one or both of those things. Finally, some substances in food have inherent antimicrobial properties, such as the capsaicin that makes the chilis spicy.

Food "goes bad" when a lot of air passes over it and microorganisms like bacteria and mold get onto it and start eating it.

When the factory puts food in jars, they seal the jars so that no air can get in. Then they "pasteurize" it - they heat it up hot enough to kill any microorganisms that might already be in the food.

So what you end up with is a jar of food with nothing alive in it and no way for anything to get in.