Here's what that means:

* Plate Boundary: The San Andreas Fault is a boundary where two tectonic plates, the Pacific Plate and the North American Plate, slide past each other horizontally.

* Transform: This type of boundary is called a transform boundary because the plates are moving in opposite directions, creating a lot of friction and stress.

This movement along the San Andreas Fault is what causes the frequent earthquakes in California, including the devastating 1906 San Francisco earthquake.