LST is low stress training. Basically, you grow the plant for a little, then tie strings around in and pull in down or to one side and exposing more stem. As it continues to grow taller/longer, you bend the stem slightly and tie those down. Basically telling it how and where to grow. This exposes the stem and makes the plant produce more leaves on the exposed stem areas, it then creates more colas and therefore making more bud sites. It's a space saving way to do it, and makes the plant really bushy.
Ive seen a few good threads describing it and showing really nice pictures. There was one with a diagram. Ill try and find it again