I've read the whole thing about Velocity in the Wiki, However it still doesn't make any sense. Thanks for the help
Velocity is telling the brick in the physics engine where to move every frame, and how far. XYZ are Left/Right, Up/Down, and Forward/Back. The way it tells which way it should go in either XYZ coord is if the value is positive or negative. Velocity can also be used; due to how physics in roblox work; to make "treadmills" by setting a velocity to a brick while it's anchored. Whenever a player stands ontop of that brick, it will add that velocity to the player. It can also be used to make trampolines.