In the First Law of Motion, we have worked with the case where no forces are acting on a mass. However, that is normally useless because we do not normally work with such cases, since such cases are so simple.
When we need to work with forces that can change how a mass moves, we will need to use the Second Law of Motion. However, before we can understand the Second Law, we have to define some terms used. The first of these is the word magnitude. Basically, magnitude refers to the size of something. Magnitude is usually given as a number, since most of the time magnitudes are defined by comparing the thing measured to a widely recognised standard. For example, the mass of something is usually given as how many times heavier it is compared to a certain lump of metal kept in France. (This is the original definition of the kilogram)
Next in the list of terms to define are scalar and vector. Simply, the difference between them is that vectors have both magnitude and direction while scalars only have magnitude. The example we are interested here is between velocity and speed. Speed is simply how fast something is moving, while velocity is speed with a specified direction.
Then, we can define the term momentum, which is simply the product of mass, a scalar, and velocity, a vector. The result is a very important value, pointing in the direction where the object is moving towards.
Finally, Newton's Second Law states that the rate of change of momentum of a given mass is exactly the vector sum of forces acting on the mass. By now, Newton's model of the universe is very much completed, and can be used to work on many problems and predict real life rather nicely.