Sound waves or vibrations are very small and air is very thin and mostly invisible. It’s easier to understand/imagine using something you can see, like big slow (compared to sound) waves in water.

Next time you’re in a pool or bath or whatever, put your hand flat on top of the water and start moving it up and down in a rhythm to make some sustained waves. Once you’ve got that going, start changing your rhythm and you’ll see that some changes make the waves bigger by amplifying the existing motion or energy in the water, and some changes make the waves smaller by going against the existing wave.

Now think about how there’s a perfect adjustment to that rhythm that will cancel out the existing wave entirely. That’s what’s going on with sound in the air that gets canceled by an “opposite” sound. It’s an opposing wave calibrated just right to cancel the other one.