The second Newton's law of motion states that the rate of change in momentum of a moving body is proportional to the resultant force.
Therefore, to understand this law one must understand the concept of momentum and that a body in motion experiences force.
As far as momentum is concerned it is important to note that as a body moves the momentum changes with change in time and also that a body at rest has 0 momentum because its speed/velocity is zero.
It is this change in momentum with time that is proportional to the resultant force experienced by the body.