Acoustic Echo Cancellation
The acoustic echo canceler (AEC) is supposed to cancel any signal in the microphone path that is correlated to the speaker signal. The microphone doesn't only pick up your voice but it also picks up the signal from the speaker, i.e., the signal from the far end. If there were no AEC, that signal from the speaker would be fed back to the far end, which is very disturbing. Since the speaker signal is available at the near end, it can be used as the input to the adaptive filter, which tries to remove any signal in the outgoing path that is correlated with the speaker signal. Basically, the adaptive filter will converge to the room impulse response between loudspeaker and microphone.