A coroutine builder is a special function in programming that helps create coroutines, which are lightweight, cooperative threads. These builders simplify the process of defining and launching coroutines, allowing developers to write asynchronous code more easily. Common coroutine builders include launch and async in the Kotlin programming language. When using a coroutine builder, developers can specify the context in which the coroutine runs, such as on the main thread or in a background thread. This flexibility helps manage tasks like network requests or database operations without blocking the main application, leading to smoother user experiences.