Undertale keeps score. It watches how many monsters you spare and how many you don't, and it shapes the rest of the game — dialogue, music, who's still alive to talk to you — around that running total. Broadly, playthroughs sort into three routes.
What you get by default: a mix of sparing and fighting, or a pacifist run missing one required story beat. Neutral has the most variation of the three, with dozens of small epilogue variants depending on exactly who lived and who didn't.
Achieved by sparing or befriending every monster you meet and resolving every boss encounter without a single kill. Widely considered the "true" ending: it's the only route that resolves the barrier, the war, and Asgore and Toriel's history in full.
The inverse: hunting down and clearing out every monster in every area before proceeding. The game notices, remembers, and does not make it easy — mechanically or otherwise — and the ending is built to make sure you feel every bit of that choice.
Routes can be mixed and are individually trackable — the game's internal flags care about specific actions, not vibes — which is why two "Neutral" runs can end up looking nothing alike.