The timecode is set to hours;minutes;seconds;frames.
If your sequence setting is 24 frames per second the timecode will show 0 to 23.
If your sequence setting is 30 frames per second the timecode will show 0 to 29.
So the 21 stands for 21 frames.
As there are : (colon) in between the numbers i gather you are in PAL-land. Pal has 25 frames per second.
NTSC has ; (semicolon) in between the numbers.
Note that you can change from the SMPT TimeCode (Hrs.;mins.;secs.;frames) to Audio Units, if you need finer adjustment for Audio, as it will be in Sample Rate, say 1/48,000th of a sec.. However, for Video, the lowest unit of time is Frame, as you cannot have partial Frames.