At the top of the Files, Albums, Projects and Employees pages there are two tabs labeled 'Filtered' and 'Selected'.
The Filtered tab displays the results that you have filtered on. For example, you might have filtered your Files with the keyword 'Mixed Use' so in the Filtered tab you will see only Files with that Keyword. If no filters have been applied, you will see all the items in your system.
The Selected tab shows you the items that you have Selected using the blue check buttons.
Selections are useful when you want to take action on those items. 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
You can read more about taking actions on your Selection here.
On the Files page, you are able to set the view options for the Filtered and Selected tabs independently. You can read more about this here.