A 2 channel radio system can control only two functions. Typically that would be throttle and turning. The left stick moves up to increase throttle, down to descrease. For turning, the right stick either controls movement of the rudder (right to turn right, left to turn left) or provides differential thrust for turning.
A typical 3 channel radio system does the same as the 2 channel but also adds up/down movement on the right stick for elevator control - climbs/dives.
Also see: What Is Trim And How Do I Trim An RC Airplane? for information on the connection between your RC airplane control surfaces, transmitter, and trim.
