What do the locks mean and how do you get rid of them?

From Joomla! Documentation

At any time you may see a padlock icon next to a specific item in Joomla!'s Administrator. These padlocks may be displayed next to content items, menu items, modules, etc.

The Joomla! system places these padlocks next to an item to indicate that a user is currently editing the item (i.e. the item is "checked out"). The lock is removed when the user clicks on the Save or Close button for that item. If the user never clicks Save or Close and instead hits the Back button or navigates to another page, the item remains locked. If a different user needs to work with an item that is in the "checked out" state, he or she must first ask a manager or administrator to check the item in before any changes can be made.

Checking Items In[edit]

There are three ways of checking items back in. One way is to contact the person that has the item checked out to see if they are done with the item. You can hover the cursor above the lock and the tooltip popup will display the name of the user who checked out the article.

With access to the Administrator and sufficient rights, you can either click on the padlock to check in only this item or click on Site  Maintenance  Global Check-in to check in all items.

The later option should be used carefully, especially in multi-user environments. This single action checks in all previously checked out items, whether they were checked out by you or not. Possible undesirable side effects may be that multiple editors end up working on the same document. In this case whoever clicks the Save button last has their version saved as the new version.