A Kingdom is the highest type of governance in Crusader Kings. The ruler of a Kingdom has the corresponding King title, and gains prestige from the number of Dukes and King Titles he (or she) holds. Like any Duke title, a King does not need to own a single piece of land in the corresponding Kingdom, it can be inherited or taken by war without any corresponding county or duke titles taken as well. However, to get a claim on a King title is prohibitively expensive (15000 prestige on the highest difficulty), and it usually easier to work your way into an inheritance through marriage or by usurping the title than spending prestige to claim it.
Like a Duke, a King can rescind titles of his vassals, and call for a general mobilization of all his Kingdom's troops, which at the Kingly level is typically very high, and wars between Kingdoms can typically involve hundreds of thousands of troops by the later game stages. Since the primary vassals of Kings are Duke instead of Counts like with Dukes however, this increases the difficulty of handling revolts, as rebellions by large and healthy Dukes can sometimes take a quarter of the Kingdom with it! Managing loyalty therefore is much more important with Dukes so as to prevent large-scale revolts of multiple vassals.
Kingly titles can be held simultaneously, although the principal Kingdom your head of dynasty is of will always be the first King title gained. That said, if you are ever able to get five King titles at once, the Papacy may see first to give you the Emperor trait, a very prestigious condition for a character.