I guess I can handle this one
Rock bolts are what is used in virtually all of roof support underground these days, where in the past you'd have timbers and beams all over the place which worked great but required an entire old growth forest to make. Without these the tunnels would collapse, end of story.
Roof support is necessary because the rock is almost always filled with joints and fractures and seams and god knows what, you are never mining though homogenous material like solid concrete. When these joint sets and fractures are exposed to the air, large blocks and slabs of rock might now be held up by nothing underneath of it. Rock bolts, believe it or not, can keep those rocks up there.
There are lots of different types, but they boil down into the same basic principle: bolts add compression force to the rock mass.
This style here works like a wide variety of resin bolts, they solidify in the drill hole and are then the face plate is tightened, this creates an area of influence around the bolt that holds that mass of rock in compression. Overlap these areas and the rock above your head stays there, and in turn forms a nice keyed in arch can hold up the rest of the earth beyond it. The quality and strength of the rockmass, as determined by a rock licker- I mean geologist, will dictate the pattern of bolts necessary and how close together they need to be to get the job done.
There are also times when there is a big 'dohney' rock just hanging up there just waiting to end you, if you can't pry it down you can pretty much nail it to the roof with a few spot bolts right though it.
These also work well in coal mines, where you typically have shale layers on the roof- thin flat layers that want to peel off and sag down. Adding these bolts can pin those many layers together and form a 'beam' of sorts between coal pillars.
There is another style of bolt that is maybe easier to understand just how it is working, its called split set. Essentially the bolt is like a pipe but with two slits down the length of it. You drill a hole slightly smaller diameter than that of the bolt, then you drive the bolt into the hole. This is like taking a cup that is loosely full of pencils and then jamming one more into it so they are call compressed against each other, enough that you could turn the cup over and they don't fall out. Or think hammering a wedge into a bundle of lumber.
That's basically it, bolts add enough lateral force to counteract gravity.