As you work in collaboration with other users, you will encounter items that are locked by other users, either because they are being edited by another user, or a draft item has been created.
Working with locked items
When a user is
editing an item, the item is locked to other users. This means other
users cannot edit the item until the current user closes the item
being edited. A lock symbol 
is displayed against items that are currently locked.
Users with administrator access can unlock items currently being edited by a user by selecting an item and then clicking the Unlock button.
Users with manager access can also unlock items so long as they have manager access to both the item and the library it is stored in.
Hierarchical item locking options
Locking of site areas, taxonomies, and categories are configurable and is not enabled by default.
When locking is enabled for site areas you cannot create any children within the locked site area. For example, if a site area is locked, you cannot create any site areas or content items within that site area until it is unlocked. This applies only to direct children of a locked parent. Items that are descendents of a child of a locked parent are not affected.
Working with draft items
When a new draft of a published item is created, a tick is displayed against the published item. This means that no other user can create a draft until the current draft progresses through a workflow and its state changes to published.