The purpose of a tech stack is to define the combination of technologies and tools used to develop and run an application. A tech stack usually comprises various layers, including the front-end (user interface), back-end (server-side logic), and database technologies. Choosing the right tech stack is crucial, as it influences factors such as development speed, scalability, and maintainability. Popular tech stacks, like MERN and LAMP, indicate specific frameworks and languages used in the development process.