Yes, as Matt L. said - it's a "click track" sent to the drummer's headphones; the rest of the band simply hears the drums in most cases and follows that.
If you need to do this, a solution I came up with that worked great for us was this (keep in mind, this will only send a MONO signal to the PA system / Front-of-House mixer but that's normally OK since you probably don't want stereo in this case anyway (or people on the left side of the room won't hear stuff on the right speaker as well, and vice versa):
Record the 'background music' on ONLY the Left side.
Record the click track (a rim shot in my case) ONLY on the Right side. (make sure you have an obvious 'count down' before the drummer should start playing).
Take the output of your playback device and send the Left side to the PA and the right side to the drummer.
We did this using a little MP3 player that the drummer controlled, running into a mini-mixer so that he had only the click in both sides of his headphones.
Buy a 'Y-Cable' that goes from mini-jack 1/8th inch (or whatever your playback device needs) to two 1/4" cables. Put those in separate mixer channels on the mini-mixer. The music on track 1 was sent out of the "FX Send" to the PA (it's volume fader was all the way down). The click was sent to track 2 and the drummer just plugged his phones into the mixer (thus getting the click in both sides of his phones and having a volume control handy without hearing the music).
Hope this helps you or someone else out there!