An automixer is like a special machine that helps control sound. Imagine a birthday party with lots of friends talking at once. It can be hard to hear just one person when everyone is talking so loudly. That's what happens when different microphones are all used at the same time.
An automixer helps make sure that only one microphone is heard at a time. It listens to all the microphones and decides which person is talking the loudest. Then, it turns up the volume on only that microphone so everyone can hear that person better.
This is really helpful when there are lots of people talking, like at a big meeting or concert. It helps make sure everyone can hear what is being said without too much noise.