This document provides an introduction to SELinux, including a brief history of how it was created by the NSA and adopted in major Linux distributions. It explains the differences between Discretionary Access Control (DAC) and Mandatory Access Control (MAC) models. It also describes how SELinux works by enforcing security policies at the kernel level to confine processes and provide fine-grained access controls, and outlines some of the key benefits it provides like auditing and strengthening server security.