Don't be mean people...
MobileMe is essentially a way to keep your contacts, e-mail, calendar, and files in sync across multiple devices. The service gives you a "cloud" that stores all of your information. So if you're on your iPhone, and you add a contact, that contact gets pushed up to your "cloud" and then the cloud pushes the contact to your other devices i.e. your iMac and your MacBook. Make a change on your Mac and within 15 minutes it will be pushed up to the cloud and then back down to your other devices.
It's still a new services and isn't very reliable as of yet. Check out apple.com for a better explanation of what I just said.