Momentum is the product of mass and velocity of an object. It has the units of kg m/s and is given as
p = mv
where, p is the momentum (in kg m/s), m is the mass of object (in kg) and v is its velocity (in m/s).
For example, a car with a mass of 1000 kg and moving at 40 m/s has a momentum of 40,000 kg m/s. If the mass of the car changes, say a passenger (with a mass of 50 kg) gets on or off, the momentum will change by 2000 kg m/s.
Momentum is also a part of Newton's laws. Newton's second law states that the net force on a body is equal to the rate of change of its linear momentum.
Hope this helps.