Timecode is nothing more than a clock that runs along with your video so that every frame has a time associated with it. It’s metadata that simplifies sound sync, conforming of online/offline proxy media, and gives a reference for broadcasters and other individuals to find a given point in your clip or program.
Timecode is read HOURS:MINUTES:SECONDS:FRAMES
Drop Frame Timecode drops two frame numbers from each minute except every tenth minute. This isn’t dropping actual video frames, just the timecode numbers relating to those frames. Notice the ; semicolon used to designate drop frame timecode.
Drop frame time code does NOT exist for 23.976 because there simply isn’t an easy way to make it work mathematically.
Frame.io has a great resource on timecode and frame rate.