What force keeps us on the ground?

Gravity, and it is by an enormous margin the weakest of the four fundamental forces - about a thousand trillion trillion trillion times weaker than electromagnetism. A fridge magnet the size of a coin holds a piece of metal against the pull of the entire planet, which is the demonstration you have at home.

It only dominates at large scale because it has no opposite. Electric charge comes in two kinds that cancel out, so a planet-sized lump of matter is electrically neutral; mass only ever adds. Pile up enough of it and the weakest force runs the universe.

Newton described what it does and said explicitly that he would not speculate about how - action at a distance across empty space bothered him. Einstein answered it in 1915 by removing the force altogether: mass curves spacetime, and objects follow the straightest available path through curved geometry. You are not being pulled down; the floor is pushing you up and preventing you following that path, which is why astronauts in orbit are weightless while very much still within Earth's gravity - they are falling freely, and missing.