When someone says a girl squirts what does it mean?

It means that when she reaches orgasm she produces a lot of fluid. There are 2 schools of thought about where it comes from. Some people think that it is vaginal fluid (produced to lubricate the vagina) that goes into overdrive, whilst others think it comes from the urethra (the tube that pee comes from) and is made up of very dilute urine and secretions from the bladder wall.

If you are with a girl who squirts then congratulations, you know she has definitely climaxed. I guess that's why so little is known about it - if you are present when it happens you are too busy elsewhere to start documenting it!