What are the curtains on the side of the stage called?

Side (masking) stage curtains This is where side stage curtains come into play. Also called masking curtains, they are used in the wings of the stage to hide (mask) equipment, gear, rigging and technical hardware that is present from the audience’s view, such as lighting bars and curtain tracks.

What is the acting area between front edge of stage and curtain?

Stage Lingo

Apron Area between the front curtain & edge of the stage
Proscenium Opening Opening through which the auidence views the play or performance. Sometimews called a picture window frame
Thrust stage An area of stage which extends from the proscenium arch toward the auidence