At the end of a compact disc, there is often a small margin allowing extra data to be written to the disc. Often in the normal writing process the data stops before this area, but software can be written to override this boundary to squeeze extra data onto the disc.
Overburning is not just a mere convenience to space-conscious home compact disc writers, but if a certain piece of software needs to be distributed that occupies 651 MB of space, overburning can be performed to alleviate the need to ship the software on two compact discs, dramatically increasing the price of shipping.