There are 6 positions in volleyball as well as 6 rotations to correspond with the positions.
In simple terms: There is one player in each position. After you side out (earn the point after the other team serves), you earn possession of the ball and you must rotate. The player in position 1 (serving position) now moves to position 6. Position 6 moves to 5, 5 to 4, 4 to 3, 3 to 2, and 2 to 1 (player who will now serve). You remain in the position you just rotated to until you side out again (this requires you to lose the serve, and once again win a point off of the other team's serve).
In more complex terms: There are 6 rotations that revolve around the position of the setter. If the setter is in "Rotation 1" he/she is in position 1. all of the other players fill in the remaining spots on the court based on position. Outside hitter #1 would be in position 2, Middle Hitter #1 in position 3, Opposite side hitter in position 4, Outside Hitter #2 in position 5, and Middle Hitter #2 (most likely a libero) in position 6. When the setter moves to "rotation 2" he/she is now in position 6. The other positions rotate as well. So now the setter is in 6, OH 1 is in 1, MH 1 is in 2, OH 2 is in 4, MH 2 is in 5. The rotaions move around all the positions in the direction you rotate (clockwise). So when you say the setter is in rotation 5, he/she is in position 3. In rotation 6 the setter is in position 2. This is the exact same thing as the simpler explanation except I'm just detailing where each designated position would be and how the formation works.