How do you use body velocity in roblox lua?
BodyVelocity is pretty complex compared to typical Objects used by scripters just starting out. The two properties of BodyVelocity that will probably concern you are velocity and maxForce.
The velocity property is a Vector3 that stores the velocity vector of the Object. Velocity is a 3D vector on ROBLOX; setting the X, Y, and Z components of this property will set the Object's velocity in those respective directions.
The maxForce property describes how fast the Object will reach the velocity property. Like velocity, it's a Vector3.
All in all, if you want a good grasp of Objects like BodyVelocity, you should try to understand basic Physics topics. Unfortunately teaching Physics is beyond the scope of this answer, so this is up to you to do independently.
You can read more about BodyVelocity on the Wiki.