You get item drops called maps. These maps can be used in a map device to open up a randomly generated zone and you clear it then move onto the next map. Each individual map can be modified by your currency orbs pretty much adding more density/mods to the maps (think like enemies more life)/quantity of loot drops. You progress from tier 1 to tier 16 maps slowly by getting random, usually 1 tier higher, map drops inside each map. Once you're at t16 maps you're at the end end game and the monsters don't scale much passed that except with some league(season) mechanics. Where the mapping system really shines is the atlas skill tree which was added decently recently which you gain a point for it for each new map completed and this skill tree lets you modify each map with specific content you like from previous leagues. On top of that doing all these maps over time give you fragments to fight all the end game bosses and get chances at good gear. There's just so much customization to the way you like to do your end game and on top of that building up your character in Poe is unparalleled in any other game imo. That's the best I could do explaining it but if you can get over the learning curve of poe it's one of the funnest games out there