Behalf means representing someone else or acting in their interest. When you do something on behalf of another person, you're doing it for them or in their place, usually because they asked you to or can't do it themselves.

If your teacher is sick and the principal speaks on behalf of her at an assembly, the principal represents your teacher's views and wishes. When you accept an award on behalf of your soccer team, you're receiving it as their representative. A lawyer acts on behalf of their client in court, speaking and making arguments for them.

The phrase on behalf of can also mean “for the benefit of” someone. If students raise money on behalf of a local animal shelter, they're working to help the shelter. When someone says “I'd like to thank you on behalf of everyone here,” they're expressing gratitude that represents the whole group.

People sometimes confuse behalf with part, but they're different. “On my part” means something you did; “on my behalf” means something someone else did for you. If you apologize on your own behalf, you're apologizing for yourself. If your friend apologizes on your behalf, they're apologizing for you, perhaps because you're not there to do it yourself.