Auditory imagery is like having a movie or a picture in your mind, but instead of seeing things, you hear sounds. Just like when you close your eyes and picture a rainbow, you can also close your eyes and imagine what different sounds might sound like.
For example, when you think about a cat meowing, you can hear the "meow" sound in your mind. Or when you imagine the sound of the rain, you can hear the pitter-patter of the drops hitting the ground.
Like a magic trick, your brain can create these sound images without any actual sounds being around you. It helps you remember sounds better and also helps with things like learning and creative thinking.
So, just like you can picture things in your head, you can also hear things in your head with auditory imagery.