Pivoting is a technique used to check that seam lines match together. It is especially useful when checking curved seams (such as armholes and collars).
To pivot, match your seams together, as if you were sewing them, and take a stiletto or a pin and place it on the point where the two seam lines no longer fit together. The stiletto (or pin) will hold the patterns in place so that you can rotate one pattern piece (normally easier to rotate the piece on top) until the seam lines match up again. Once they are in line, you can remove the stiletto (make sure you hold the patterns in place) and move it to the next point where the seams no longer match. Continue the process until you reach the end of the seam.
For a detailed example of this technique, you can have a look at this tutorial.