Ability Modifier - the modifier (see below) associated with your ability score
Ability Score - a measure of innate abilities combined with training and competency for that ability. The average human has a score of 10 or 11 in most of these, while a score of 0 in any of them implies death. The abilities in DnD are Strength, Dexterity, Charisma, Wisdom, Intelligence, and Constitution.
Advantage - this is a bonus to an action. When you have advantage, you roll 2 d20s instead of 1 and take the higher result. It is negated by disadvantage.
Charisma - an ability score measuring force of personality
Class - the training a character receives which determines their abilities, strengths, and weaknesses.
Constitution - an ability score measuring endurance
Dexterity - an ability score measuring agility
Die - the singular form of the word dice. These are usually referred to as d[number of sides], for example a six-sided die would be called a d6. Dungeons and Dragons uses d4, d6, d8, d10, d12, and d20. A d20 is the most common die to be rolled.
Disadvantage - this is a hindrance to an action. When you have disadvantage, you roll 2 d20s instead of 1 and take the lower result. It is negated by advantage.
Dungeon Master (DM) - a person who organizes and oversees the playing of a TTRPG by narrating the details of the story that are not controlled by the players. See also Game Master
Modifier - a number that changes a die result or natural roll. These may be positive or negative and are impacted by training, armor, magical items, spells, and even damage a player has taken.
Non-Player Character (NPC) - any character in a game that is not controlled by a player; in a TTRPG, these are controlled by the Game Master
Player Character (PC) - a fictional character tabletop role-playing game whose actions are controlled by a player rather than the rules of the game.
Proficiency Bonus - a bonus added to skill checks, saving throws, or attacks for skills that a character is proficient in.
Race - The humanoid species found in the Dungeons and Dragons universe. These range from humans, dwarves, and elves, to the more unique like ratfolk, owlins, and many more
Game Master (GM) - a person who organizes and oversees the playing of a TTRPG by narrating the details of the story that are not controlled by the players. See also Dungeon Master
Hit Dice - The number of fice rolled to calculate how many hit points a character or monster begins with. These are also used to roll to heal during short rests.
Hit Points - the amount of "health" your character or an NPC has
Intelligence - an ability score measuring reasoning and memory
Level - a numerary value to describe a PC or NPC's experience and strength.
Strength - an ability score measuring physical power
Tabletop Role Playing Game (TTRPG) - a roleplaying game (RPG) conducted through discussion, generally around a table. This is as opposed to something like a Live Action Role Playing Game (LARP).
Wisdom - an ability score measuring perception and insight.