yes yes, I know...a very stupid question when you know what it is, but the fact is I DONT know
I've seen it's mentioned all the time and it must be basic equipment, but I havent found the explanation of what it is. I've tried a search here, I've tried a search on google, ... no help. It must have something to do with the radio equipment, but what is it? Otherwise I'm never gonna learn

Thank you

RX=receiver (the radio part that goes in the plane.)
TX=transmitter (the radio that you hold in your hand to control the plane.)

In case your wondering, "crystal" is what determines what frequency (channel) your radio and receiver transmit/receive on. You need one crystal for each and, you guessed it, they both better be the same channel or they won't be able to talk to/hear each other! Usually you pick a channel for your radio, and then buy a matching crystal for all the other receivers/planes you happen to get in the future.

thanks! at least now I know what ppl are talking about when tx/rx is mentioned

and crystals are sometimes referred to a X-tals... FYI