Simple linear regression is a statistical method of assessing association between two continuous variables.
If one of the variables is clearly an outcome, and the other an explanatory variable, simple linear regression allows you to predict the outcome for different values of the explanatory variable.

Karafa MT, Rybicki LA. Biostatistics and Epidemiology. In: Walsh D (ed). Palliative Medicine, Saunders Elsevier, Philadelphia, 2009. p. 159.