Feature scaling is a technique used in machine learning to standardize the range of independent variables or features of data. There are several common feature scaling methods including standardization, min-max scaling, and mean normalization. Standardization transforms the data to have a mean of 0 and standard deviation of 1. Min-max scaling scales features between 0 and 1. Mean normalization scales the mean value to zero. The document then provides the formulas and R code examples for implementing each of these scaling methods.