Drop frame timecode is a legacy of the introduction of color television. Due to a complication with the introduction of chroma subcarrier components to television signals it was deemed necessary to reduce the frame rate of broadcast video by 0.1%.
It’s important to understand that no frames are actually dropped; rather the frames play back at the slightly slower frame rate of 29.97 frames per second instead of 30.
What are in fact dropped are some of the frames being counted in the timecode associated with the video. In order to keep regular time in sync with the video streams and still count 30 frames in each second, roughly 2 frames of video are dropped from the timecode counter for every minute of footage.
Video can be measured in drop-frame and non-drop frame. To denote the difference, semi-colons separate drop-frame timecode notation, while colons are used for non-drop.
Drop-frame persists in the modern era of 24 frame per second HD video. To maintain compatibility with the legacy broadcast systems, video cameras frequently shoot at 23.98 frames per second instead of 24.