No problem, this hobby has a load of info and I remember how confusing it all was when I started, so happy to offer some advice if I can.
The frequency is just that, one or more bands that the radio operates on. 915mhz is better at longer range, but 2.4Ghz is abit more universal (as its used on most small drones etc)…
The main thing to care about here is that if you go for a 2.4Ghz ELRS system, you also need to use a 2.4Ghz receiver, and same for 915, or 868mhz.
As for the radiomaster, 2.4Ghz an CC2500 are actualy slightly confusing here, as if you are referring to our product page, we are reffering to the radio transmitter module in the radio itself, as there are two options.
The 2.4Ghz one is an ELRS module, and the CC2500 is actualy a different module not compatible with ELRS, but rather some other older radio control protocols such as Frsky, and Futaba SFHSS. Its runs yet another open source radio firmware which you can learn about (or confuse yourself with at first) here - pascallanger/DIY-Multiprotocol-TX-Module: Multiprotocol TX Module (or MULTI-Module) is a 2.4GHz transmitter module which controls many different receivers and models. (github.com)
In general, if you are unsure, for now I would say just go with ELRS as I think overall its becoming more and more the default protocol to use on FPV drones.