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.