The process of combining several tracks together and re-recording them onto another track is called bouncing. This is normally done to free up tracks for more recording. For example, you might have three background vocals recorded on tracks 1 to 3. By combining these tracks with a mixer, and routing them to track 5 (for example) tracks 1 to 3 can be erased and used to record other materials. Keep in mind that bouncing does require that you pre-decide on levels, EQ, etc. for the tracks being bounced – once they are combined, it is impossible to adjust their relative levels (overall level and EQ of the bounced track can, of course, still be adjusted).