AC: Armor Class. Represents how difficult a character is to hit. The bracketed number is the total of an attack roll, after all modifiers, necessary for a target to be struck by an attack.
Attack Roll: The total of a d20, plus any applicable modifiers. Used to determine whether an attack is successful. Also called a to-hit roll.
BHB: Base Hit Bonus. Added to all attack or to-hit rolls.
Cleric Spell: A magical, holy effect available only by Clerics.
d20 or d6: Refers to what type of die to roll. d20 is a 20-sided die. d6 is a six sided die. A d20+2 would be the result of a 20-sided die plus two. d6-1 would be the result of a six sided die minus 1.
Damage: The amount to be subtracted from a target's hit points when they are successfully attacked or affected by some spells.
HD: Hit Dice. Always a d6. HD 1+1 means 1d6+1; HD 2-1 means 2d6-1, etc. Total of all rolled HD determines a character or monster's Hit Points.
HP: Hit Points. Represents a character's current health. When reduced to 0, that character or monster is dead.
Level: Typically this a measure of a character's experience and general prowess. When referencing spells it reflects a spell's complexity.
Level Gain Factor: Used to determine how many adventures a character class/race combination needs to advance to the next level.
Magic-User Spell: A magical effect available only to Magic-Users.
Melee: Any hand-to-hand attack, whether armed or not. Target must be adjacent to, or within 10 feet of attacker.
Missile: Any projectile weapon, whether shot or thrown. Target must be within listed range of missile weapon.
Move: How quickly a character can travel. Sometimes called movement or movement rate.
Round: 1 minute. Combat is measured in rounds.
Save: A character's ability to avoid danger. Roll a d20, plus any applicable modifiers. Equal to or above Save is a success. Sometimes called a Saving Throw.
Total Adventures: The number of adventures a character needs to complete to gain a level.
Turn: 10 minutes. Many spells have their duration in turns.