Refractometers measure the amount of sugar in a liquid (wort in this case), they do not function properly when alcohol is in solution. The refraction is different when alcohol is present, so they do work before any fermentation takes place but afterwards your results will be incorrect. You have to use a correction formula to get an accurate reading after fermentation has taken place.
Every refractometer is different. You actually need to compare the reading on your refractometer and hydrometer over multiple batches to come up with the Wort Correction Factor for your refractometer. I've read that using 1.04 is a good average correction number to use however, but you won't be 100% sure unless you do the leg work yourself.