The hit point total given is the average hit points for the monster's hit die. These are the assumed 'norm' for a generic monster of its type. Some of the Bestiary's ancestor books - the Monster Manuals - actually stated that the stat block is an example of a weak monster of that type, but no word on if Pathfinder follows the same idea. Either way, the hit dice you're seeing are indeed the monster's hit dice, and can be useful if you decide you want to roll or maximize their hit points. They also help you track things like the creature's skill points, feats, and level-based effects.
Some creatures take up more, or less, space than a Medium-sized creature. The "space" entry indicates how much space the creature takes up, typically in a square.
Advancing a monter's hit dice is one way to advance its CR, though not necessarily the best way. Pathfinder introduced Simple Templates - like Advanced and Giant - that can boost a creature's CR by 1 or 2 very quickly and with a minimum of fuss. Adding templates in general can make a creature more powerful, and you'll note that templates come with a suggested CR boost.
Making it Stronger or Weaker Without That
You can also make a creature stronger - or weaker - by changing the feats it's selected, swapping its equipment out (or adding some if there isn't any), altering its skill selection, and/or raising or lowering its ability scores.