Valve springs close the valves after they have opened to either let in the air/fuel mix or the exhaust fumes, so the piston doesn't collide with the valve.
Coil binding occurs when the spring is compressed so much that the coils contact each other, which can lead to complete valvetrain and engine failure.
High Performance Valve Springs
Valve springs need to be able to push the valve back after being opened, and do so before it gets hit by the piston, so a high performance valve spring needs to be able to do this better than a stock spring. This can be done with a stiffer, longer, or additional spring. When a longer spring is used, sometimes (depending on the spaces made available from the engine design) some of the spring seat location needs to be ground away, allowing for the larger spring to compress without binding.