Tha Manual covers most of these, or maybe all.

Open Doors: percentage to unlock doors. I think this one is the STR-derived bashing locked doors open, not the thief ability.
Locked doors have a difficulyt-level which you have to be at, or at worst be in 5% range to be able (save is always your friend). 6 here is equal with 6% on the thief ability, so it's mostly negligable.
Up till 18/XX STR-based Open Door can open SOME extra random chest (and doors) to you, but 19 is the real magic number, especialy in the first game (and 25 in BG2/ToB).

Reaction and Reaction adjustment is covered in the Wiki.
It's a semi-complicated thing derived from alignment, reputation and charisma.
Some quests to pick up or to finish for a certain reward needs you to have a certain reaction-level. This mostly will trouble you at the very start of the adventure, or if your CHA is VERY low (in BG2 right at the start you get a 18 CHA item, so the fuss stops there).
The more important part is, if you don't start Good Alignment, you can't have the +1 Dagger in Candlekeep, and at the waterfall with the resurrectable cat you can't have the anti-undead scroll if you are like below 5-6 CHA. So I wouldn't worry too much.

Missile Adjustment is the bonus for you to hit things with ranged weapon. It is a technical data, what you should look at, is the THAC0, actualy, that's the final number in that calculation. This is the number you can check if the ranged weapon is not selected when you look at the sheet. Ranged stats are pretty forgetable, as the main tactic is to stay out of range wearing Boots of Haste, and eventualy the target will fall. if the bullet can actualy harm it.