Twerking is a form/type of dancing in which the rump (bum, butt, booty, rear end, etc) shakes and moves to the rhythm of the music.
Most music is made up in quarter notes, I.e., each beat can be counted 1 - 2 - 3 - 4. When you twerk, you skake your rump in what is known as "double-time" or eighth notes.
Begin by counting the beat in quarter notes as described above. Then double that, counting 1 and 2 and 3 and 4 and, etc. These are the beats that you shake your behind on.
When twerking, you can move your butt up and down or side to side or a combination of the two.
Check YouTube for Miley Cyrus twerking. She may not have much of a bum, but she's got the rhythm.