The front edges of stair treads are commonly rounded over into a half-round configuration. This makes them both more aesthetically pleasing as well as less likely to injure someone walking up them. It also reduces the incidence of catching the front edge of the step with your foot and tripping.
Because it somewhat mimics the shape of a bull's nose, these rounded edges are commonly called bullnoses. While wood can be purchased with the bullnose already completed, it is possible to add your own.
Anchor the wood to be finished to a firm work surface and secure with work clamps.
Install a half-round bit into your router. The radius of this bit should be half the thickness of the wood you will be working.
Put on your safety glasses and run the router along the edge. Move the tool slowly and use caution when approaching the end of the wood to avoid splitting.
Remove the clamps and turn the wood over. Reinstall the clamps.
Run the router along the opposite edge. The front edge of the wood should now be a half-circle.
Sand the edge with a fine-grit sandpaper to obscure any tool marks. It is difficult to put a bullnose on stair treads already installed. If you do not have a router, round the edges with a belt sander or palm sander. It is difficult to get a consistent radius on the curve with this method.