When you put something on a back burner, you make it a low priority. In other words, you've decided that the task or activity on a back burner isn't immediately important.
The phrase back burner can be used to talk about things that have been deliberately put aside so more important or urgent things can be taken care of. You might say that you've put your exercise program on a back burner until you can get caught up on writing your novel, for example. This term comes from the cook's habit of moving pans to the stove's back burners so he can focus on dishes that need more immediate attention.