In After Effects, the anchor point is the geometric reference and rotation center point of a layer. Above is an example of how changing the anchor point to the corner of a rectangle. When this rectangle is rotated, it rotates around the corner instead the default center. Each layer in a composition has an anchor point that can be changed in a number of ways.

1) The anchor point property dialog can be accessed by clicking on the anchor value in the Time Layout Window.

2) The anchor point can be moved visually in the Layer Window if display is enabled in the controls pop-up. Use shift-option to snap layers or anchor points when repositioning. Also, you can use the info window to keep track of just how far you go.

3) The Pan Behind tool on the tool palette can change the anchor point directly, but it moves the layer position as well -- and moves the mask, even the default mask, if the anchor point center was not selected carefully.

The best description of the The Pan Behind tool I've seen is by Trish Meyer on AOL's After Effects forum. See Trish's regular columns in DV Magazine.