Can someone explain what is Rigidbody.GetRelativePointVelocity(Vector3 relativePoint) ?

Don't use "GetRelativePointVelocity" as it contains an error and is unusable. I've explained the issue over here in the bottom half..

Unity's "GetRelativePointVelocity" method actually mix local and world space velocities. It calculates the localspace tangential velocity based on the angular velocity and adds the worldspace linear velocity. The result just makes no sense at all.

The concept of a local space velocity only makes partially sense. When you have a rotating motion the velocity would constantly change direction / magnitude if it's combined with linear velocity. Rotating motions are really hard to grasp. If you have a wheel rolling downhill, from worldspace point of view it looks like the bottom point is standing still, then accelerates to twice the linear velocity when the point is at the top and then slow down back to 0 when you are back at the bottom point. However in a rotating system things can look quite different depending on the observer. From inside the wheel (local space) a point on the surface would not move at all, obviously, since it's part of the wheel and the particles the wheel is made of does not disintegrate itself. That's why local space velocity just makes not much sense for a rotating body.

So if you're looking for the world space linear velocity on a local point, using "GetPointVelocity" with a local point that is converted to worldspace is the way to go. Though if you want something else you have to be more specific what and why.