All of us have studied regressions or at least heard about them in newspapers or journal articles. Regression lines are a visual representation of the regression equation. Gareth, Witten, Hastie, and Tibshirani in the book An Introduction to Statistical Learning, have defined a simple linear regression as, "it is a very straightforward approach for predicting a quantitative response Y on the basis of a single predictor variable X. It assumes that there is approximately a linear relationship between X and Y". Readers should refer to chapter 3 of that book to learn the concept of linear regression and its application using R.
From the preceding plot, we observe that horsepower has a negative relationship with miles per gallon (mpg).