Answering machine detection is the name for the process that a calling system uses to decide whether a call was answered by a live person or by an answering machine or voicemail system.  Calling campaigns often hang up on calls that are detected as answering machines, and only pass the "live" calls out to the phoners.
Answering machine detection is not an exact science.  The system listens to the sounds as the call is answered and analyzes things like the amount and type of background noise and the sound qualities of the voice on the other end, to make a judgment on what type of answered call it is.  This means that when you use answering machine detection, there will always be some answering machines that are passed through to the agents, and some live calls that are hung up on by the system.