Momentum is the product of the mass and velocity of an object. It refers to the quantity of motion that an object has.
A 50-g ball rolls across a table at 2 m/s.
Momentum = .050 kg (2m/s) = 0.1 kg m/s
A 100-g rolls across the same table at 1 m/s.
Momentum = .100 kg (1 m/s) = 0.1 kg m/s
The momentum of the two systems are equal as we can see the values above.