Ahead of means in front of something or someone, or before something happens in time.

When you're ahead of your friend in line, you're standing closer to the front. When a runner is ahead of the pack, she's leading the race. Picture a relay race where one team's runner crosses the finish line while the other teams are still running: that first team is ahead of the others.

The phrase also describes being before something in time. If your birthday is in March and your sister's is in June, yours comes ahead of hers. When a teacher assigns homework that's due ahead of the test, she means you need to finish it before the test happens.

Sometimes ahead of means understanding or preparing for something before others do. A scientist who sees problems ahead of other researchers notices them first. A chess player who thinks ahead of her opponent anticipates moves before they happen. Being ahead of schedule means finishing earlier than planned, which usually means things are going well.

The opposite is behind, whether you're talking about position in space or time.