With this feature enabled, your Facebook friends can tag you and "check you in" to a place. You receive a notification when you're tagged, and an update is posted on your wall telling your friends where you are and who you're with. You can remove the tag at any time.
To disable the feature, visit Account > Privacy Settings > Customize Settings. Scroll to the middle section—"things others share"—and click "Edit settings" next to "Friends can check me in to Places" to disable it.