What does "AFOL" mean?
AFOL stands for "Adult Fan Of LEGO".

From an unofficial LEGO acronym glossary:


  AFOL
  
  Adult Fan of LEGO. The most common term used to refer to LEGO fans who are adults. Generally pronounced to rhyme with “hay-foal”.


Or from Brickipedia's glossary (also unofficial):


  AFOL - Adult Fan Of LEGO