I'll try make it easy to understand.

When you catch covid-19, the virus starts reproducing and growing in your body. It takes your body a long time to develop the little soldiers (called antibodies) that kill the virus.

When you have the little soldiers, the next time you're exposed to the virus, your body will kill it much faster because it knows how. Over time, your body forgets how to make the little soldiers.

If you're exposed to a tiny bit of virus, you may not even get symptoms because they haven't reproduced enough to cause problems.

However, if you're exposed to a large amount of virus, you could get serious effects because more of the virus can reproduce before the little soldiers can destroy it.

A vaccine is a dead virus (I know many different types exist, this is a basic explanation). It's giving you a controlled amount of the virus that does not reproduce so your body can naturally build little soldiers to fight the virus if you come into contact with the live virus.

There's no real difference between getting covid-19 and many types of the vaccine for it.