Locks or unlocks a file or folder on the server. Checkout locks prevent other users from checking out items in other workspaces, ...

Locks or unlocks a file or folder on the server.   Checkout locks prevent 
other users from checking out items in other workspaces, and checkin locks 
prevent users from checking in any pending changes on an item.

tf lock itemspec /lock:(none|checkout|checkin) [/workspace:workspacename]
        [/recursive] [/login:username,[password]]
        [/collection:TeamProjectCollectionUrl]