Think of a mixer as a telephone switchboard. Remeber on the old western movies, when the local operator wopuld patch each call through to their respective party line?
Well, an audio mixer takes many individual signals, and joins them together. Then the mixed signals are sent to a new location.
ie:
You have one amp, but 4 singers.....
Ch1-Larry
Ch2-Moe
Ch3-Curley
Ch4-Shemp
**each of the four mics gets plugged into their own channel of the mixer, then a single output ( or 2, in stereo, if that's called for ) gets routed, or plugged into the amplifier.

That way 4 mics are "mixed" together as they enter the audio path into the amp.

Does that help?