In rock climbing each movement has a degree of technical difficulty. There are a few different big classes of climbing that effects how climbs are graded: trad/traditional is long(ish) climbs where not falling to your death involves jamming tiny bits of metal in cracks and using them as an anchor point when you fall, sport where someone bolts anchor points into the rock and you clip into them as you go up and bouldering where the rocks aren’t that high and you can just place a mat to fall on to. In trad the grading for a climb combines technical difficulties of individual moves from one holding point to another combined with difficulty of placing safety mechanisms/danger of death. In sport the grading mechanism combines the technical difficulty of the hardest single move from point to point with some measure of endurance required to get to the top. In bouldering because the climbs are very short it is extremely focused on how difficult each movement up the rock actually is with rarely any other factors entering into the equation. There are multiple grading mechanisms for each style and some favour different things, this is just a rough idea of how many of them shape up. It is less about particular styles or types of moves required but more about the difficulty of each single progression along the climbing route and sometimes the risk involved, where long climbs factor the difficulty of doing many fairly hard moves, short climbs such as bouldering are usually a small number of typically difficult movements. Difficult in this case because they require a combination of strength, balance and flexibility, particularly finger and core strength with generally a high power to weight ratio on the body in general. It has been about 15 years since it was something I was big into, so I guess some things might have changed but this is the essence of it.
This is a reasonable example of bouldering (I watched without sound so no idea if anything offensive may be there) Off the Wagon 8B+, Val Bavona - YouTube … 8b+ is pretty difficult grade.