A tech stack refers to the combination of technologies used in the development of software applications. It typically includes languages, frameworks, libraries, and tools for front-end and back-end development, such as JavaScript, React, and Node.js. Understanding a tech stack is crucial for developers as it influences the application's performance, scalability, and maintainability. A well-defined stack allows teams to utilize the best technologies suited for specific tasks, ensuring efficient collaboration and development processes. Ultimately, the choice of tech stack can significantly impact the success and sustainability of software projects.