Porous is referring to the density of the live rock. Good live rock is very porous so that a rock actually weighs a lot less than it would look. The more porous, the more surface area for bacteria to grow which is one of the major biological filters in our tanks. Also, the more porous generally the easier to mount corals and such to the rock, the more places for inverts and fish to hide.
Calfo's book "Reef Invertebrates" gives a great description of live rock. Some rock is formed from dead acros and you can tell by the shape, some from dead staghorns and some of all kinds of dead corals.
Florida Live Rock (aquacultured now) has a lot more life but a great deal of it is not necessarily desirable.
Lee