Several variations of Newton’s Second Law exist.  Each says essentially the same thing, but modifies its expression slightly.  I list these here to give you a little bit of historical context.  Don’t worry if you don’t understand them yet.  In later chapters we will revisit some of these interpretations.  You can read a more elaborate background on wikipedia at Newton’s Laws of Motion

The Original Latin and its Early English Translation

In the original Latin, Newton wrote…

Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.¹

Motte translated this in 1729 as…

The alteration of motion is ever proportional to the motive force impress’d; and is made in the direction of the right line in which that force is impress’d.²

Early 20th Century Interpretation

In slightly more modern language, James Clerk Maxwell interpreted this as…

The change of momentum of a body is proportional to the impulse impressed on the body, and happens along the straight line on which that impulse is impressed.³

Unfortunately, most modern textbooks reserve the topics of momentum and impulse until later chapters.  We will revisit this statement in later courses.  For the time being, you can think of momentum as the product of the mass and velocity of the object.  You can determine the impulse by multiplying f applied force by the amount of time it is applied.

¹ Newton, Isaac, Mathematical Principles of Natural Philosophy, 3rd Latin edition, 1726

² Newton, Isaac, “Mathematical Principles of Natural Philosophy“, 1729 English translation based on 3rd Latin edition (1726), volume 1, containing Book 1

³ Maxwell, J.C. Matter and Motion, Sheldon Press, New York, 1925