When you view a folder by clicking on the folder name on the left, the contents of the folder display with one item selected (highlighted). You can select one or more items to perform actions as follows:
Click to select one item at a time. Clicking on another item de-selects the previous one.
Double-click to open an item.
Right-click an item to display a menu of actions that can be applied to this item. You can also right-click after selecting multiple items, to apply the same action to all of them at once.
Control-click selects multiple items. Can also be used to de-select the current item, leaving nothing selected.
Shift-click selects multiple items in sequence. Click on one item and then Shift-click another item further down the list and all items between the two selected items are highlighted.
You can select multiple items, and with one right-click on the mouse choose to Mark as Unread, Tag, Delete, or Move all selected items.