Vrms - how does it work exactly?

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In the Devialet Configurator, I've noticed some inputs have this Vrms setting. can someone explain, in layman terms,  how this works and what my real world results would be if i made changes to this setting? I'm assuming this would raise or lower the volume (gain) to my speakers at a given level (like a boost so to speak).

Broadly speaking your assumption is right.  The max input level stated in Vrms ("root mean square Volts") is a measure of how big the input signal needs to be for the amplifier to produce its full power output.  As you adjust the max input level it changes the internal gain of the amplifier.

If you have connected something like (say) a CD player to a line-level input, then the CD player might produce for example 1 Vrms when the CD is playing at its loudest.  If the Devialet is configured with its input level set to 1 Vrms then it will produce its full output when the CD is at its loudest.  If you increased the Devialet's input level to 2 V rms then it would not produce full output when the CD was playing at maximum; on the other hand if you decreased the input level to 0.5 Vrms then the amplifier would reach its maximum power before the CD was playing at full volume, and might "clip" (run into a temporary overload) when the CD was at full volume.  You'd see that on the Devialet display as it goes orange or red (I can't remember) when the input signal gets high enough to put it into overload.

To summarise, a bigger number in the Vrms field means that the amp is less sensitive and will sound quieter for a given input level.  A smaller number makes the amp more sensitive so it will sound louder for a given input.

does the rms setting need tomatch the Speakers or the input device? curious if you guys use this setting and your outcome. 

The max input level setting should be chosen to match the input device -- in particular, its output level which will normally be part of its specification.  Typically most analog sources other than phono cartridges have so-called "line level" outputs, which are usually around 1 Vrms at full volume.  Phono cartridges have much lower output levels, usually a few hundred microvolts (μV) or several millivolts (mV), which you'll see if you configure the phono inputs.  On the whole it's better to start with higher numbers in the configurator's Vrms field (lower sensitivity) and then reduce the number (increase the sensitivity) if you need to, being sure to avoid clipping.