Gravity and Mass

A Rigidbody hands its object to Unity's physics, as it did when you added one in Add a Component. From then on gravity speeds the object up by 9.81 m/s every second, and the Rigidbody has the settings that decide how. Two of them are mass, in kilograms (1 for a new Rigidbody), and useGravity (on for a new Rigidbody).

Mass does not change how fast things fall. Gravity pulls a heavy body harder, and the heavy body is harder to speed up by exactly as much, so a 1 kg crate and a 50 kg anvil dropped from the same height land on the same frame. Mass matters when bodies push each other: the anvil knocks the crate aside, and the crate hardly moves the anvil. This party balloon gets a light mass and is left alone by gravity, so it hangs where the scene put it:

void Start()
{
    Rigidbody rb = GetComponent<Rigidbody>();
    rb.mass = 0.2f;
    rb.useGravity = false;
}

With useGravity off, the balloon still has its mass and still moves when something bumps into it: only gravity stops pulling. Set it back to true and gravity pulls again from wherever the object is. There is no air in Unity's physics either, so a feather falls like a brick unless you give its Rigidbody some Linear Damping.