The Theory of relativity that Einstein developed involves two separate theories. There’s Special relativity and there’s General relativity. It’s simple so far but as you’ve guessed it gets more complicated.
When an observer moves very fast time progresses more slowly for that observer. Why does that happen? You see energy has mass. Yes you knew that, it’s the e=mc2 stuff. Energy of motion is a type of energy. When something moves very fast the energy of motion adds to its mass so it can’t change so easily. But it is changing; its position is changing. Well it can’t change so easily in other respects so time slows down. If you want to learn more try reading Theory of relativity from Wikipedia and linked articles.
- Special Theory of Relativity, Albert Einstein Video helping to explain Special Relativity in fairly simple language
General relativity says that objects don’t fall because of gravity. They fall because space time is curved. Einstein worked out any number of very complicated equations to explain all that. Time passes more slowly in gravitational fields. Light bends in a gravitational field as if it were going through a lens. There’s more. Try reading Theory of relativity from Wikipedia and linked articles.
- Einstein's General Theory of Relativity Explained Video helping to explain General Relativity in fairly simple language
None of this makes sense to you. Well let me assure you the clever scientists who wrote the Wikipedia article are sure it makes sense.
These four videos can help you understand relativity.