Perhaps it would be helpful to remember that Baldur's Gate is based on the old game called Dungeons & Dragons, which has lots of charts that determine your chance of success whenever you try to do anything, and in which you roll dice to see whether you succeed. UncleScratchy gave a pretty good description of THAC0, but I also think that you should try to imagine yourself rolling a 20-sided die when you try to hit a monster to get the feel for Dungeons & Dragons.
To keep it simple, the lower the THAC0, the better. For example, if you need to roll an 18 or higher to hit a monster (a 15% chance of success), you're not going to be as successful as someone who has to roll a 4 or higher (an 85% chance of success). See how that works?
You can also keep in mind that the amount of damage you do in one hit is also determined by a roll of the dice. For example, if a weapon does "1d8" damage, that means it does one to eight points of damage. To determine how much damage you inflict on a successful hit, you roll "one eight-sided die", which is where you get the "1" and the "d8" in "1d8". By the same token, if you use an item that does "6d6" damage, the damage is calculated by rolling a six-sided die ("d6") six times and adding up the total.
Baldur's Gate is a visual game, of course, and the computer does all of the dice-rolling for you. But hopefully, this will help explain the meaning of the jargon to you.
"Backstabbing" is when a Thief sneaks up behind an enemy and stabs him in the back. Since the enemy is unprepared for it, the Thief can inflict "double damage", "triple damage", etc. For example, if a Thief normally does 1-6 points of damage with a short sword, you would double that if he has a backstab multiplier of 2x. Backstabbing only works if the enemy can't see your Thief, so you need to "Hide in Shadows" or drink a Potion of Invisibility before you try to sneak up behind an enemy.
I'm glad you enjoy the game. If you have any more questions, don't hesitate to ask. We enjoy the game, too, and we like to help.