What does it mean if your premonitions or dreams come true?

Having "dreams" and premonitions is remarkably normal. Most people experience them now and again. There is an old saying "Even a stopped clock is right twice a day."

There is nothing sinister or spooky about having the occasional premonition.
The content of the premonition may be a clue here as well.
If you "see" a friend or someone you know well, doing something fairly normal, winning a prize, passing a test or getting a new boyfriend, then the likelihood of that happening is pretty high.
If you "see" your neighbour winning a million dollars and getting into a car accident on the way to collect, the odds on that happening a pretty high and you may be getting real premonitions.

The best way to test this out, is to start a "dream" journal.
When you "see" something, write it down in as much detail as you can, then forget about it. Use a separate page for each "sight" and be sure to date them carefully. When something happens that is similar to what you "saw", go back to that page and compare your prediction with what actually happened.
Write the whole thing down and make references to your prediction.
One or two of these a year that are "close" isn't much of a pattern.
If however you get "close" on several, and "dead on" on a few, all happening on a weekly or even monthly basis, I would say you may be a precognitive.

Your next step is to figure out what you want to do with it. Precognition is a double edged sword, walk it carefully.

It is not always easy or comfortable (for you or for those around you) to know what's going to happen.
Sharing your "sight" can get you labeled as a "nut-ball", people don't really believe you can do what you say you can. And even the very best of us aren't always right.

We regularly face the problem of what to do. Do we say something or keep our mouths shut.
If you warn someone of a possible difficulty, they may do something different and avoid the danger. Then neither you or they will ever know for certain if you were right.
If you don't speak up and they get hurt, you end up feeling guilty.