Hi, the switch you are referring to is on the front face of the TX on the left. It is a "dual rate" switch. When it is off the servo travel is reduced so any control movement is "smaller" and the heli will react more slowly to cyclic movements for example. When it is on and the "AER" is seen on the graphic it is set to "larger" servo travel and so smaller stick movements are needed to get the same result as before. Its good to start with it "off" but once you are more confident to turn it on. It becomes almost easier to fly because you have a wider range of control.

Hope this helps

PS the switch on the top left is the idle-up switch and has three positions: Normal, Idle Up 1 and Idle up 2. As a beginner its best to fly with the switch in the normal position and leave Idle up for later.