Trim is basically a fine tuning function for your controls. Since models, whether helicopters or airplanes, rely upon control surfaces to be aligned in a certain way, trim adjustments allow you to make small adjustments in order to keep certain settings.
For example, in the case of airplanes (I am more familiar with airplanes), you need to adjust the trim settings on your radio transmitter so that your left and right ailerons allow the airplane to fly level. You will need to trim your left/right control to achieve this balance. Likewise, you will need to trim your elevator control so that your model flys level and does not climb or fall.
Once your model is "trimmed" it should be able to fly in a straight and level line. This is what is meant by "trimming" an RC airplane.
I apologize as I am not as familiar with helicoptors, but the same should hold true for helicoptor settings. In order to attain a stable hover, you would need to trim various settings.
If you ever want to find out more about RC airplanes, please check out www.economyrc.com, blog.economyrc.com, and support.economyrc.com.