Explain what a suspend function is and how it works in Kotlin.

A suspend function in Kotlin is a function that can be paused and resumed later without blocking the thread. It is used for writing asynchronous, non-blocking code, for example, for network operations or long-running tasks.

Such functions are declared with the keyword suspend. Inside a suspend function, you can call other suspend functions, which allows writing sequential code that runs asynchronously under the hood.

Example:

suspend fun fetchData(): String {
    delay(1000) // pauses execution for 1 second without blocking the thread
    return "Data received"
}

Here, delay is a suspend function that pauses execution, allowing other coroutines to run during this time.

Thus, suspend functions help write asynchronous code more simply and cleanly, avoiding callback hell.