Dungeons & Dragons is a game of imagination. There are rule books. The Player’s Handbook has an explanation of races you can choose to be and classes. These include elves, half elves, humans, giants, halflings, dwarves, gnomes and other fantasy make believe races. When you start a game you choose a race based on well based on benefits of that race. Some have faster travel speeds or can see in the dark.... elves don’t sleep they meditate and can live hundreds of years.
Then you choose a class, this is also based on what you want out of the game. You are basically building a person. During the course of the game you will act as though you were this character. If you choose thief as your class.... well you might choose to randomly pick pocket someone. How? I’ll get to that :)
The players choose characters while the DM (or GM, Dungeon Master or Game Master respectively) creates the world. When you play the DM will describe what your characters see, smell, taste, feel and you will tell the dungeon master how your character reacts.
After choosing a race (lets say half elf) and a class (you gonna be a thief for this cause it’s what I can think up right now) you’d roll a number of die and assign those totals to ability scores. These ability’s will determine how your character acts in situation along with dice rolls. The abilities are Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. I love the tomato explanation so I’ll put that here. Strength is being able to crush a tomato. Dexterity is being able to dodge a tomato. Constitution is being able to eat a bad tomato. Intelligence is knowing a tomato is a fruit. Wisdom is knowing not to put a tomato in a fruit salad. Charisma is being able to sell a tomato-based fruit salad.
When you assign a number to those abilities and using the information in the handbook you get your modifiers. These are the numbers you add to rolls when determining what you do/how you do it.
So, you are a half elf thief and I say that you and your friends (other player characters) are hanging out in a bar. I as the DM ask you to make a perception check as you look around the bar. If you roll low (on a 20 sided die) I will tell you that you see people at the bar. If you roll well I tell you that you spot a particularly rich looking chap. You can tell because of his fine cloths and the heavy purse you spot on his right hip. He seems particularly interested in a beautiful elf female in the corner. Very distracted, distracted enough that as a thief you might be enticed to do what you do best. So YOU decide if you’d like to try to take his coin. You choose not to that’s fine we move on to whatever else might be interesting.... but this is your sample story for here and now so you are gonna steal this purse. So, you do a little sneaky sneak and get up next to richy rich. I as the DM would roll for Richy to see if he notices. If he dose I would tell you he grabs your hand. Maybe he makes a show for the rest of the patrons. Let’s say he doesn’t notice though and you attempt to snatch his loot. You would have a bonus to try because of your class. As the DM I would set a difficulty level. Like if he had it tied super tight or if he felt the weight lifted. You’d roll, add the modifier and see what happens. Lets say I set the difficulty at 12. If you rolled 11 and added a +2 for the modifier you’d get the money. If you rolled a 7 and added that modifier you would not get the money.
During the course of the game as the DM I might describe and item you come across and keep. Some thieving gloves perhaps… and those would give you a higher modifier.
All of this is made up all descriptions and imagination. You level up based on whatever the DM decides and gain abilities planned out in the player’s handbook. Game session generally last 4 hours and full games can last years! You basically live out life as an imagined character in a made up world. There are guidelines for both DM and players set out in books. There are monsters and pre-made adventures in the world but so much of it can just be made up that “homebrew” is not an uncommon style of play.
FYI I’d love to put an owlbear in the cave if they choose the component quest but I think it would kill them. I intend on starting them at lvl 1 .... I do not know how I intend to level them up yet.