The term 'tech stack' refers to the combination of technologies, frameworks, and programming languages used to build and run a software application. This stack can include components for the frontend (user interface), backend (server-side), and database management. Choosing the right tech stack is crucial, as it influences the performance, scalability, and maintainability of the application. Common tech stacks include MERN (MongoDB, Express.js, React, Node.js) and MEAN (MongoDB, Express.js, Angular, Node.js).