To forestall means to prevent something from happening by taking action before it occurs. When you forestall a problem, you stop it before it even starts, like bringing an umbrella to school when you know rain is forecast, forestalling the problem of getting soaked on the way home.

The word suggests smart, early action. A teacher might forestall confusion by explaining tricky instructions carefully before students begin their work. A soccer goalie forestalls a scoring opportunity by rushing out to grab the ball before the striker can shoot. Countries forestall conflicts by signing treaties and agreements before tensions escalate into war.

Notice that forestalling requires awareness and timing. You can't forestall something that's already happening. You have to see it coming and act first. A student who studies hard the night before a test forestalls the panic of being unprepared. A park ranger might forestall a wildfire by clearing dry brush during fire season.

Today it simply means getting ahead of a problem through foresight and quick action.