The 525/60 line/field format used with the NTSC colour coding system does not run at exactly 60 fields per second but 59.94, or 29.97 frames per second — a difference of one in 1000.
Time code identifies 30 frames per second.
Drop frame time code compensates by dropping two frames at every minute except the tenth.
Note that the 625/50 PAL system is exact and does not require drop frame.