On the left pane, under the RECENT section is an item Flagged. The number tells you how many photos you have flagged and clicking on Flagged collects all your flagged photos and shows them in one window.
Flagging photos you want to only mark temporarily is easier (and cleaner) than making a new album and dragging photos to it or assigning Keywords. Once you are done with the photo you flagged, select it, and click Unflag from the bottom toolbar.
The keyboard shortcut for Flag / Unflag is Command-. (Command-Period). Using the shortcut key instead of going to the bottom Toolbar every time can really speed up your workflow.
If you want to remove all the flags at once, hold down the Option key and choose Photos on the Menu Bar. One of the items on the dropdown menu is Clear All Flags. (If you don't hold down the Option key, the dropdown menu will say "Unflag Photo".)