When a plant gets rootbound, it basically means there's nowhere for new roots to grow. You can check very easily by carefully removing the contents of the pot, plant and dirt...to do this, gently squeeze the pot on all sides to loosen the dirt a bit, then place your hand over the pot with the stem between two fingers and tip it upside down...the contents should slide out easily. If you see a lot of roots circling around the outside of the dirt, your plant is probably rootbound and you should transplant.
I hope this helps, but if I'm not being clear then please don't try it, there is a chance you could damage your plant if you do something wrong.
I probably should have asked this first...what size container is the plant in and how long has it been in that container?