Time code with ; is drop frame time code. Ready to get your mind all messed up. It counts hours; minutes; seconds;and frames.
Because drop frame timecode is only for NTSC footage that runs at 29.976 frames per second instead of 30 to keep the time code reporting the correct time, which is essential for broadcasters and other folks that must work to an exact time, frame numbers are occasionally dropped so you go from frame number 28 to frame 0 skipping frame number 29... That's the general idea. NO frames are dropped. You don't loose any video, you just have to adjust the display so that 60 minutes is actually 60 minutes.
If the separator is : then you don't drop frame numbers but the time displayed by the timecode is not accurate when the frame rate is not a whole number. The longer the video, the more the time display is off. You don't have this problem at all unless your frame rate is 23.976, 29.97 of 59.94 fps. At any other frame rate the time display will always be accurate. You can thank color television and the FCC for this mess. Broadcasters could not figure out how to slip the color information into the signal and still deliver 30 frames per second so they slowed down the frame rate to give time for a couple of more lines in the blanking part of the signal, the part you don't see, to be sent to the TV to tell it that it should be showing you a color picture.
Frames are just that frames. It doesn't matter what the frame rate of the project is. If you have 300 frames at 30 frames per second your video is 10 seconds long. If you have 250 frames at 25 frames per second then your video is 10 seconds long. Any time the frame rate is a whole number and you have timecode display selected you will see the correct time display in timecode.
If your timeline is showing 10f then you are looking at 10 frames. If there is punctuation then it is most likely timecode. There is another option that is feet and frames but you have to be working with film to have than matter and not many folks do that anymore.
Type "timecode" in the search help field at the top right corner of AE to learn more. There are a lot of community resources that will help you figure things out.