This tool allows you to rotate your editing canvas to see things from a different angle or to position your image at a more natural angle for editing e.g. when you are using a brush tool to follow a curve.
Press the R key to activate this and show the rotate symbol.
Click and drag the cursor to rotate the image. Pressing SHIFT at the same time rotates in 15 degree units.
The toolbar at the top has a Reset button to bring the image back to normal, or you can set a precise angle.