I keep thinking it means that I can plug it directly into my power supply instead of running it though my RAMPs 1.4; is that correct?
Here is the link to it, 200*200mm Square Silicone Heater Pad.
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"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.
 I just noticed you've mentioned "mains voltage"
DON'T DO THAT
If there is 12V on your heatbed and your power supply gives 12 V then you can do it but there will be no control over your heating procedure.
How does it work?
RAMPS is connected to Arduino and then through Arduino to your computer. So applications can say to Arduino - hey, heat it up up to 70°C. So Arduino sends this command to RAMPS and RAMPS is switching on heating, meantime it measures temperature and sends this measurement to Arduino so it knows if your heatbed is already at 70°C or not. Having that knowledge Arduino can decide to still heat it up or rather switch the power of the heatbed off.
If you connect your heatbed directly to your power supply then all of the above controlling sequence will not take place.
Imagine to connect your lightbulb directly to the wall plug. You can do it but then you will not have any chance to switch it on or off. Right? ;)
So in my opinion - don't do that - don't connect your heatbed directly to your power supply.
The heater pad described in your link is designed for 12 VDC or 24 VDC. Therefore, the term Mains Voltage in the description is misleading and probably wrong. Read the voltage and power requirements again and it appears that the heater pad can be connected directly to the RAMPS 1.4 without an external relay or SSR.