Single-click is an optional setting that changes the way files are selected with the mouse. It makes files behave kind of like links on a web page - files are opened with a single-click instead of a double-click, and you can select a file simply by hovering over it with the mouse. Some users prefer this way of working as it reduces the number of mouse clicks (especially double-clicks) that you need to do.
Single-click can be enabled with the Single click to open an item option on the File Displays / Options Preferences page. This option has no effect in Power mode, but in all other view modes, the behaviour of the mouse is modified as follows: