Slime is the trade name for a glue that they put on the inside of the tubes. Theory is that it will start to ooze out through any puncture and quickly reseal the wound. My friend swears by it for mountain biking through the cactus needles in Arizona. He says if you get a flat you just pull the sharp thing out, spin the wheel to fling the slime to the outside of the tube, and pump it back up. That said, it makes the inside of the tire pretty nasty through repeated punctures, and I've seen them blow out and cover my friend in, well, slime. Some people love 'em, some people hate 'em.