var mass: float;
float mass;
mass as float

The mass of the rigidbody.

You should strive to keep mass close to 0.1 and never more than 10. Large masses make physics simulation unstable.

Higher mass objects push lower mass objects more when colliding. Think of a big truck, hitting a small car.

A common mistake is to assume that heavy objects fall faster than light ones. This is not true as the speed is dependent on gravity and drag.
	rigidbody.mass = 0.5;
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        rigidbody.mass = 0.5F;
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		rigidbody.mass = 0.5F