I want to ask a dumb question...what is a "nit" player exactly??

I've found 10NL on FTP and Bodog were the easiest for me to make money. I've had, what I thought, alot of "bad beats" on FTP, but overall I was up 6x my deposit when I just cashed most of it out at christmas (not great, but I don't play alot on there and my deposit bonus had just expired).