Backset: This is the horizontal measurement from the edge of the door to the center of the hole bored through the door. On a lock or latch this is the distance from the front of the face plate to the center of the door knob hub.
The backset of the lock or latch needs to match the backset of the pre drilled hole in order to install the hardware without modifying the door. Tubular latches for conversion doorsets are usually 2 3/8" or 2 3/4" backset. For interior mortise locks, the most common backsets are 2 1/4", 2 3/8" and 2 5/8". For entry mortise locks the backset is usually 2 1/2" or 2 3/4".