The Clone brush G copies pixels from one part of the layer to another, like a rubber stamp.
Hold Option and click the point you want to copy from. This sets the source and paints nothing.
Move to where you want the copy and drag. Pixels are copied from the source, keeping the same offset for the whole stroke.
The copy is exact—including transparent pixels—and is sampled from a snapshot taken when the stroke begins, so painting across your own source doesn’t feed back into itself. The source point is remembered between strokes until you Option-click a new one.
Brush size and shape set the stamp footprint, and pixels whose destination colour is protected are skipped.