A Folder Compare session provides commands, such as Copy, Move, Delete and Synchronize, to help you reconcile differences on the folder level. It also is a platform for spawning file views to understand and reconcile differences within files.
Actions generally require a selection. You can select specific files or folders from either or both sides and then perform some action on them. (If you want to synchronize the entire contents of the base folders, consider using a Folder Sync session instead.)
To select items in the Folder Compare view
Click on an item on either side of the display to select it. You can click in the center column to select both sides at once. Hold down the Ctrl key and click to add items to your selection. To select a range of items, press the mouse button over the first, swipe to the last, and release the button. If your swipe crosses the middle column, entries on both sides will be selected.
To select all items on one side
Pick Edit > Select All 
(or Edit > Select All Files 
). Hold down the Shift key and press the left arrow key to limit the selection to the left side. Press the right arrow key instead to limit the selection to the right side.
To compare files with different names
Click the first file, hold down the Ctrl key and click the second one, and then pick Actions > Open. To use the keyboard instead, position on the first file and press the space bar, then position on the second one and press the space bar.