Your soul... Just kidding.
They just want your time, effort, some skills (if you have any specific ones) & a bit of passion!
From farming to web developing, whether you speak one language or several, use your skills to help out your host.
Be a part of the staff and dedicate a few hours a day giving a hand to your host.
Be pro-active and willing to learn. Make a positive impact on the people you meet, wherever you travel.