Mass in general relativity

Imagine you have a very big trampoline that is perfectly flat. If you put a small ball in the middle of the trampoline, it will stay in place. But if you put a bigger ball on the trampoline, it will create a dip in the trampoline where it sits. If you then put a smaller ball near the bigger ball, it will roll towards the bigger ball because of the dip it created in the trampoline. This is similar to what happens with mass in general relativity.

In general relativity, mass creates a dip in the fabric of spacetime. This dip is called gravity. The bigger the mass, the bigger the dip in spacetime and the stronger the gravity. So, just like how a bigger ball creates a bigger dip in the trampoline, a bigger mass creates a bigger dip in spacetime.

Objects, like planets, stars, and even you, move through spacetime and are affected by this dip in spacetime created by mass. This is why you feel gravity pulling you down towards the center of the Earth. It's because the mass of the Earth is creating a dip in spacetime that causes objects to fall towards it.

So, in general relativity, mass is the thing that creates gravity, which is the force that pulls objects together. And this happens because mass creates a dip in spacetime, kind of like how a big ball creates a dip in a trampoline.