Open almost any 2D game and look at the ground. Grass, brick, water, sand. Look closely and you will spot the same little picture repeated again and again, like bathroom tiles. That is not laziness. It is one of the oldest and smartest tricks in game making, and it is called a tilemap.
A tile is a small square picture, often 16 by 16 pixels. A tileset is the collection of tiles a game can use: one for grass, one for dirt, one for water and so on. The tilemap is the level itself, and here is the surprise: it does not contain any pictures. It is just a grid of numbers. Say 1 means grass, 2 means dirt and 3 means water. A row of the level might be stored as 1 1 1 2 2 3 3.
When the game draws the screen, it walks through the grid, looks up each number in the tileset and stamps the matching picture in the right place. The numbers say what goes where. The tileset says what each thing looks like.
Pick a tile below and paint on the grid. Watch the numbers underneath: that grid of numbers is everything the game would need to save for your level.
Small files. The counter above shows the gap. Storing one number per cell takes a tiny fraction of the space needed to store a picture per cell. A level of thousands of cells is a few kilobytes. The tileset is drawn once and reused everywhere, which also keeps the game light on memory and fast to draw.
Fast level building. Making a level means painting, not drawing. Want a lake? Pick water and drag. Want to change every wall from brick to stone? Swap one picture in the tileset and the whole level updates, because the map only says "tile 4", not what tile 4 looks like.
Easy rules. Because the world is a grid, the game can attach rules to tile numbers. Water slows you down, stone blocks you, sand is fine to walk on. To check whether you can step right, the game just looks at the number in the next cell. No complicated shape maths needed. Many engines let you mark each tile as solid or not once, in the tileset, and then every use of that tile follows the rule.
Real games usually stack several tilemaps. A ground layer holds grass and paths, a decoration layer holds flowers and rocks, and a front layer draws over the player, like the tops of trees you can walk behind. Each layer is its own grid of numbers, so you can change one without touching the others.
Tilesets are often combined into one big image called a sprite sheet or atlas, because drawing from one image is quicker than juggling many. And some engines can auto-tile: you paint "grass" and the engine picks the right edge and corner pictures for you, so borders look neat without hand-placing each one.
You do not need to code a tilemap from scratch. Popular engines such as Godot and Unity include tilemap tools, and there are free standalone editors such as Tiled that save levels as data files engines can load. A good first project is one screen with four tiles: ground, wall, water and a goal. Keep the tiles small and the grid simple, and you will have a playable level in an afternoon.
Next time you play a 2D game, look for the repeating squares. Once you see the grid, you will notice it everywhere, from tiny puzzle games to huge worlds.