The Point Speed Restriction Rail (PSR) is a track piece that effectively says to trains “Go at below or equal to this speed at this point, do whatever afterwards”. Some uses include:

• Making a more complex station rail, where a luaATC can set internal and external information, or lines
• Cargo tracks, which require a speed of <3 to do their operations

To use, place, and right-click. You will see a dialogue, where you should set the maximum permissible speed in nodes per second.