The Common Traffic Advisory Frequency (CTAF) is a frequency designated for manned aircraft pilots to communicate with each other directly, air-to-air, while operating to or from an airport without an operating control tower.
And remember that on a Sectional Chart, it’ll actually say what CTAF frequency you should be using. The CTAF is always identified by C / circle symbol, and the frequency always appears ‘before’ or to the ‘left’ of the circled C.
The control tower frequency is identified by the letters CT and the frequency appears to the right of the letters.
The tricky thing is that often, the CTAF frequency and CT frequency is the same! You’ll see an example of this in the FAA’s test supplement legend, in the Airport Data explanation section.