A "neat freak" is someone who keeps their home really clean and organized.
"Neat freak" sounds a bit negative, so you should avoid calling someone else a neat freak if you're not close to them. A more positive way to talk about someone like this is to call them "neat":
You're so neat.
Some other phrases using "freak" are:
a control freak
a fitness freak
All of these mean that the person does something much more than other people.