Red is right...effacement is the thickness of your cervix. When you are 0% effaced it means it is still really thick and there is no sign of labour. When you are 50% effaced it means your cervix is half as thick as it was before and shows signs that your body is getting "ready". When it's 100% effaced it means you have no cervix left (thickness is completely gone) and at this point you are usually dialated some if not all.
When I was 5 cm's dialated I was still only 75% effaced so I still had 25% of it left (thickness wise). Hope this is of some help. You can always check it out on the net!
Just to add a point if your a first time mom its actually a better judge of labor by effacement. Actually they say the more effaced you are the more likely you are to go into labor. But your already at four anyway so you really almost there, literally. My doc told me if I got to four he would consider me in active labor.