Complex sentence: The general sum operator\\ sum can then be considered as a ternary function symbol taking a lower bound value, an upper bound value and a function to be summed-up; it is called a second-order function symbol.
Split into simpler sentences: The general sum operator\\ sum can then be considered as a ternary function symbol taking a lower bound value, an upper bound value and a function to be summed-up. Due to its latter argument, the\\ sum operator is called a second-order function symbol.