To the right hand side of the Files, Albums, Project and Employees pages there is a bar of buttons that enable you to take action on items. The buttons that are visible on the Action Bar will depend on your user permissions.
Sometimes these buttons work specifically with the Selection that you have chosen and sometimes you will have the option of working with the items in the Filtered view instead. Buttons that only work with Selections will be deactivated when your Selection is empty.
Examples of actions you might want to take include:
Drag multiple selected Files into a document
Edit your selected Files in one go
Generate documents based on your selected Files, Projects or Employees
Share your selected Albums