It’s a Haskell list comprehension. You read it as: The sum of x for x from one to 999 where x mod three is zero or x mod five is zero. well, the bit in square

It’s a Haskell list comprehension. You read it as: The sum of x for x from one to 999 where x mod three is zero or x mod five is zero. well, the bit in square brackets is a comprehension. `sum `is just ordinary function application.