there are people who know a lot more about the technical side of things than me... but i like to keep things simple so here is my explanation.
Just like a guitar string has to be tuned, so does a bowstring. Each bow is different and requires a different brace height to match the balance of the bow being shot from a given draw.
Brace height is the distance from the deepest part of the grip to the string. By finding this sweet spot you will find the bow is dramatically quiet, has less hand shock, and possibly more accurate.
There are some people that will get an arrow close to tune then change brace height slightly up or down to achieve perfect arrow tuning. There are some the prefer a specific brace height and forgo the whole bow tuning process.
personally, when tuning a bow i start at a point I know is definitely too low, shoot a few arrows and increase brace height by increasing twists in the bowstring until my bow becomes almost silent. I then add silencers and start tuning arrows. The lower the brace height the more power is placed on the arrow because the arrow is in contact with the string longer. A higher brace height can be more forgiving for the opposite reason.
there is a start I'll let someone else step in now...