It's one of those things that's complex at first but when you wrap your head around it, it seems so obvious.
Just read up on Einstein's theories of relativity. In this context, it's more important to read up on general relativity, not special relativity.
Basically put, 100 years ago Einstein showed that mass bends space and time. Add a large amount of mass, that adds a large amount of gravity, that bends not only space but also time. Like a bowling ball on a tramampoline. Anything that moves closer to the ball will be pulled toward the ball. But time is linked with space so if you are the object moving toward that bowling ball, then time slows down for you.
This is the part that really messes with your head, but we have confirmed it. If you take two atomic clocks and put one at sea level, and the other on the tallest mountain, then leave them there for 1 year. When you check the clocks, they will be different. Both clocks should be the same but one clock will move slightly faster than the other. It's pretty much imperceivable and insignificant to humans, but one clock will be nanoseconds ahead of the other because the pull of gravity is stronger at sea level than it is further away (like the top of a mountain). The stronger gravity is, the slower time moves.
Take this to the next level by adding in an extreme mass. Like a black hole at the centre of our galaxy. It's a mass made up of 100's of millions of stars, but it's a black hole so it's also a tiny point. Like the point before the Big Bang, but smaller. But it's influence is based on the amount of mass and the fact that it's made up of the mass of millions of stars means that if you went to the centre of the galaxy, you would be sucked into the black hole but time would slow down so much that before you were sucked right into the middle, you would see 100's of billions of years pass around you.
Yeah, I don't explain it very well and it's complex and takes a while. But when you get your head around it, it's insane. And it's stuff that Einstein hypothesised 100 years ago, and humans have since confirmed to be 100% true.