Pacing is learning to find the right balance between spending time on an activity without pushing yourself so that it significantly worsens your pain. Pacing is the opposite to just pushing through and finishing an activity and then being in so much pain the next day that you cannot cope with anything at all. It is about breaking activities up into small increments. It is about definately stopping when you start to feel pain or increased pain, depending on your situation. Pacing can help prevent flare ups and keep you mobile. Some people who are trying to pace themselves find it useful to set a time limit and also a rest time or a change of activity.
Here is a site that has helpful worksheets that can help you with pacing: PSYCHOLOGY TODAY