Yes great resources already posted. In a nutshell though it means to shine a bright light into the egg through the shell around day 7 and 14 to see the developing chick inside.
If they have no changes you know the egg will never hatch. By the time they are ready to hatch if you go somewhere...
I had my broodies lone egg hatch day 25 so don't give up yet. Day 26 is pretty much the latest I think I've read of them hatching though.
did you candle them at any stage and know they were developing?