Momentum is the measure of how difficult it is to change an objects' velocity.
Momentum can be increased by increasing the mass of the object or the velocity of the object.
e.g. a lorry going at 80km/h down a hill with a very heavy load would have a big momentum. - It would be very difficult to stop! (If this was an exam question you would have to convert hours to seconds and km to m)
The formula for calculating momentum is:
Momentum (kgm/s) = mass (kg) x velocity (m/s)
Conservation of momentum just shows that momentum is never lost but just transferred from one object to another e.g. if the big lorry runs into a car then all the momentum will be transferred to the car.