All 15 Iterators & the Iterator Protocol questions
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What exception signals the end of iteration, and how does a for-loop use it internally?

Beginner

Answer

StopIteration signals that an iterator has no more values. A for-loop repeatedly calls next() on the iterator behind the scenes and automatically catches StopIteration to end the loop cleanly, without the exception propagating visibly to your code.
it = iter([1, 2])
while True:
    try:
        print(next(it))
    except StopIteration:
        break
Real-world example Understanding this lets you manually drive an iterator with next() and a try/except when you need finer control than a for-loop provides, such as peeking ahead.

Common follow-ups: What happens if StopIteration is raised inside a generator function in Python 3.7+?;How is StopAsyncIteration different?

Async Generators & Async Context Managers;Exception Handling
All 15 Iterators & the Iterator Protocol questions