Total Blocking Time (TBT) is a web performance metric that measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) during which the main thread is blocked for long enough to prevent user input responsiveness. Specifically, it sums up all periods when tasks take longer than 50 milliseconds to execute, indicating delays that could make a page feel sluggish or unresponsive. Lower TBT values generally reflect a smoother, more interactive user experience. This metric is commonly used in tools like Google Lighthouse to help developers identify and optimize performance bottlenecks.