To forewarn someone means to give them advance notice about something dangerous, difficult, or unpleasant that's coming. When you forewarn people, you're letting them know ahead of time so they can prepare or protect themselves.

If your teacher forewarns the class that Friday's math test will be especially challenging, she's giving everyone extra time to study. Weather forecasters forewarn coastal cities about approaching hurricanes so residents can evacuate safely. An older sibling might forewarn a younger one that their mom is in a bad mood today.

It's different from a regular warning because it emphasizes the advance notice: you're getting information early enough to actually do something about it.

People often say “forewarned is forearmed,” meaning that knowing about a problem in advance helps you prepare to face it. If you know the trail gets muddy after rain, you can wear the right boots. If you know your friend gets cranky when hungry, you can pack extra snacks. Being forewarned gives you the advantage of preparation.