Slacklining is done by tightly stretching 1" - 2" high strength nylon webbing between two anchors (usually trees or posts). Everyone's method for set-up differs; I use a carabiner pully system to get mechanical advantage in the system, while some people opt for an easier to use racheting come-along to crank the line tight.
After the line is sufficiently tight, the goal is to get up on the line (standing, sitting, kneeling, whatever you what!) and walk it, do tricks on it (spins, flips, poses, etc) and just have fun in general.
Slacklining has a very steep learning curve, as it is basically a form of tight rope walking, coupled with trampoline gymnastics. This dynamic blend makes even just an hour of casual slacklining an extreme core workout!
Check out this video of the Gibbon slacklining team doing their thing!