It's a muscle spasms in your lower back. Spasms are contractions of muscles. The area usually feels really, really tight, and sometimes it might feel like you have a knot in the area. If you push on the spasming area, it will probalby also feel very hard. Spasms can be quite painful, as well, and they can last a long time. What it is...is your body's way of protecting an area it thinks it injured in some way. The muscles around the area go into spasms and hurt, which prevents you from moving in whatever direction. It's a signal to your brain to be careful because something is wrong, though it doesn't necessarily have to be something bad or severe. One can just bend or twist the wrong way, or pull somethign in there..and then get spasms in the area.
