What does "Uses Mains Voltage" mean on this 200*200mm Square Silicone Heater Pad?
"Mains voltage" is the alternating current provided by your wall outlets (eg 120v in USA). In order to use a mains voltage heatbed in a 3d printer, you will need to wire up your controller to switch an AC SSR. That way, your low-voltage-DC RAMPS board can control AC current. 

If you plug a heatbed directly into a power supply, it will never stop heating, and may burn itself up or even cause a fire. Don't do that. 

Because mains voltage has enough voltage to kill you, it should only be used for STATIONARY components, such as the PSU supply wiring and Delta heatbeds. With moving beds, particularly Mendel/i3 style Y-beds, wire fatigue and rubbing can expose the electrical conductors and create a serious safety hazard. 

200x200mm is a very reasonable size for a DC bed heater, so there's really no good reason to use an AC heater here. It's more complex and less safe. I recommend getting a different heater.