This tool inserts a 2D shape into the surface of the mesh. It works by splitting edges and triangles to form the shape in the mesh.
Click the Stamp button to open the stamp flyout. Click a shape to choose one. Then click + drag on the surface to add the stamp. The cursor position at the click-down point defines the center of the stamp, and the amount you drag defines how large the shape will be.
When you release the mouse, a stamp is created in the mesh. A new face group is assigned to the faces inside the stamp shape (makes sure you have “Group Color” selected in Mesh Color Mode in the preferences window to visualize this.)
To create a new stamp shape, select set of triangles in an object and go to Select > Convert To… > Convert to Stamp. Your selection should have only one boundary loop of edges.