Time to First Byte (TTFB) is a web performance metric that measures the amount of time it takes for a user's browser to receive the first byte of data from a web server after making an HTTP request. TTFB includes the time spent on DNS lookup, establishing a connection, and server processing before any content begins to load. A lower TTFB indicates a faster server response, which can improve user experience and search engine rankings. Monitoring and optimizing TTFB is essential for ensuring efficient website performance.