I will try to explain:
Let us go back to 1600s when force is not defined quantitatively, and try to understand as to how we can arrive to the definition.
Force: a feel of push or pull is named by humans as "Force".
Lets imagine one situation in which we have a toy gun which is pushing a small marble kept on an ice frame.
The following things happened:
1. Now the toy gun pushed the marble, using single arrow.
2. We plotted the instantaneous rate of change of momentum for the object on y axis and on x axis, we plotted Force.
Now mind here that the force is not defined quantitatively yet; But intuitively, we can say that the toy arrow is applying a push on the marble (thus its applying a force on the marble).
Therefore on x axis, we plotted force as using one toy shoot.
Now, we used two toy arrows at the very same time and applied onto the marble.
Again we plotted rate of change of momentum on y axis, and on x axis, we can intuitively think that if ine toy arrow can provide some value of force, now two toy arrows will be providing twice that amount.
Therefore, we plotted that on x axis.
We did similar experiments with 3 toy arrows, 4 toy arrows etc and plotted rate of change of momentum vs this pushing force on y-x axis respectively.
When we joined all these, for the marble of constant mass, it formed a straight line, passing through zero.
Thus one thing is clear that the force acting on a body is directly proportional to the rate of change of momentum.
F directly proportional to d(mv)/dt
Now to eliminate the constant of proportionality, we defined 'Newton' as a unit of force such that when we apply a force of 1 N, its capable of producing the acceleration of 1m/s2 for a body of 1kg mass.
If on the other hand, lets say we defined the unit of force as Ron, and defined that one ron is that force which will produce an acceleration of 0.5 m/s2 for a mass of 1kg, then we can write F=2ma.
But for the time being till, unit of force is Newton, lets enjoy with the following equation