Auditory masking is when one sound blocks or covers up another sound in your ears so you can't hear it. It's like when you're trying to listen to someone talk to you, but there's a lot of noise going on around you, and you can't hear what the person is saying very well because the other noises are too loud.
This happens because your ears can only handle so much sound at once. If there are too many sounds, they all try to get into your ears at the same time, and it becomes hard for your brain to separate them and figure out which one is important.
For example, imagine you're watching a movie and the music is really loud. If someone tries to talk to you, you might not be able to hear them very well because the music is covering up their voice. That's auditory masking in action!