When a projector is not exactly on a level plain with a projection screen the projected image is not symmetrical. Most of the time the projector is lower than the screen so the top of the image is wider than the bottom of the image and looks like a"keystone."
Keystones are the top stone in an archway that holds the arch together sort of like a wedge. Many tripod type screens have the ability to tilt forward at the top which adjusts for the keystone effect. Look for a notched outpiece of metal that points toward the audience at the top of the screen where the screen hooks when it is unrolled from its holder.
Note: You are usually stuck when the screen is mounted on the wall or drops down from the ceiling
Trick for ceiling mounted screens -- Tie a heavy weight to the bottom of the screen and pull it back, which has the same effect as tilting the top forward. Many LCD projectors can adjust for keystone electronically. Make sure you read the onscreen menu, or heaven forbid, "the manual" to learn how to use this function.