If you play low limit poker then you have undoubtedly encountered many tight poker players. They are also often referred to simply as nits.
A "nit" in poker is somebody who plays extremely tight. They only play very strong hands before the flop, and usually if they are betting after the flop, then they have something very good as well.
Learning how to smash the nits is absolutely essential for your success in today's micro stakes games. And the reason why is because they will be your most common opponent by far.