As bcworkz mentioned, you have a partial blockage that needs to be cleared.
In order to clear this, you need a snake. These come in manual and powered versions of various lengths and diameters.
How long of one you need will depend on how far down the pipe the blockage is. I believe 25ft is the length of the most basic units but 50ft is also common (and obviously as you get bigger and powered units, the length and diameter increases). If you have an accessible clean-out, this is the best place to start from. Alternatively, the easiest (but not best) place to start from is after the P-trap under your bathroom sink (ie: remove the trap, then snake). In the event that you don't have a clean-out and are unsuccessful from the sink, you might need to remove the toilet and start from there.
You might also be able to rent a larger unit from your local home improvement store.
Using it is relatively simple - you feed the flexible cable down the pipe, and use the handle to turn spin the cable while holding the handle to get around bends and to break up the obstruction once you hit it. The one thing to watch for is using too small diameter of a snake for a large obstruction as this will just punch a hole in the obstruction but not actually clean it out.
Some models have different attachments you can use (root cutting, grease, etc.) but the one pictured is the most basic/common one you'll see.
A plumber might use a water jet to help clean it out too.