Before you try this function you must have attributes from a table selected (in other words highlighted in yellow.
Your selection will be cleared from the map and you will receive a notification in the information Panel.
If you have more than one feature from different layers selected on the map you will be able to choose which layer's selections you wish to remove.
Please note You Cannot Undo a Clear Selection.
Recommendation: unless you want to print a map with features selected (they usually appear shaded yellow but could be usefully deployed to highlight a particular attribute/feature that you wish to emphasise) it is a good idea to hit this button before you create a print page.