BALANCE (first bar/number) is the correlative to accuracy with the weapon/magic. The higher it is, the heavier a weapon you can use while still retaining a high hit%. Additionally, the base hit% is identical to the balance level itself. It is raised through attacking normally, by using skills, or by being hit while defending.
AGILITY (second bar/number) is the correlative to evasion. The base % is 2/3 the agility level itself. It is raised through avoiding attacks, or by being hit while defending.
SKILL (third bar/number) is the correlative to critical hits. The base % is 1/3 the skill level itself. It is raised at a slow, fixed rate by attacking normally, and much more so if you land a successful critical hit, or by being hit while defending.
Magic is raised in these three ways as well, including critical hits. However, while weapon skills only take weapon weight into account, Magic takes both your weapon weight AND armor weight into account. While it is entirely possible to be a successful mage in a full set of plate mail, dual wielding Dwarven axes; actually getting to this point is another story entirely.
As for how the base and max percents work, it's sort of a general success range. While you might start out with a 1-99% hit rate, that just means you have anywhere between a 1% and a 99% chance to hit something. Although it averages to be more along the lines of 75% in that case, as the maxhit is used twice in the calculation.
Something else I did was force it so you can only level up once at most. Any excess experience is removed entirely. This is to prevent grinding for years to get your weapon skill to level 99 in one fight.