To add to Ariel's comment:
By default, when using the web client, all content that is created or modified is automatically locked in the GUI. That means that other users when browsing content in their sandbox will see a small padlock icon meaning that the item is currently sitting in another user's modified items list and is currently not available for editing. This resolves a concurrency issue prior to 2.1, where if an item was modified but not yet submitted another user could theoretically make concurrent, and conflicting edits.
Only user's who created and modified an asset can edit that asset in their own sandbox (since they own the lock). The exception to this is a Content Manager, who can also edit locked resources in their web project in that user's sandbox. If you want to release the lock, you can do so by either Undo-ing your change (selecting the Undo action and reverting to its original state) or by submitting the asset (which auto-unlocks once approved and checked in to Staging).
Please note that for power users (developers), locks are not automatically taken for actions done via the CIFS interface. In future releases, for these developer types working via CIFS will intend to expose more of the repo services for branching and merging, which currently is not exposed in the web client.
Kevin
