It is J's 'insert' function. It inserts the verb on the left between the items on the right. I think I've see it called 'apply' for J too, and I think 'reduce' as defined in other functional languages would be correct; it's just that J calls it 'insert'.
(reduce + (range 1 100001))
but you can make yours a little nicer by relying on a Range being enumerable, hence:
(1..100000).inject(:+)