To record all the data on a CD-R disc one track at a time. The laser is turned off after each track, which produces a two-second gap. The track-at-once feature, which was added shortly after the first CD-R drives were introduced, enables songs and data to be written at different intervals. The lead-in sectors that are written at the beginning of each session contain the location where additional data can be placed on the disc.
For audio CDs, the session can be left open in order to mix songs from different sources, but it must be closed in order to play the disc. If the disc is closed, no more songs or data can be written to the medium. See disc-at-once
This dialog box from Adaptec's EZ CD Creator software shows disc-at-once and track-at-once options before "burning" a disc. The "Leave Session Open" option at the top is ghosted in this dialog, because a data CD has been selected. Were an audio CD being recorded, it would be an available option.