bouncing is another way of saying trackout. It has, as laurend points out above, additional idea that it is processed and reflects any edits that may have been made.
the term comes from a time when the only way to make more room in a multi-track tape setting was to bounce tracks down to a stereo pair - you bounced everything you could out to two tracks retaining mix levels and panning and eq and effects that may have been applied/added at that stage, then started recording new material to the now vacant tracks.
Do not confuse this with bouncing out the stems of a project - generally the stems will be clean - no eq, no fx, no panning - so that they can be worked with by another mix engineer/producer