Very basically, trim is used to adjust the control surfaces so that the aircraft will fly "hands off". On modern aircraft this is usually done with some sort of trim wheel operating an adjustable "trim tab" on the control surface. Trim can usually be adjusted in all 3 axes these days - pitch (elevator), roll (ailerons) & yaw (rudder). The pitch trim is the most important & varies throughout the flight envelope. There are different trim settings for take off, climb, cruise & landing.