What is the q @ symbol doing in the context of being applied as a function call?

What is going on in the example here?

Source: http://code.kx.com/q/ref/select/#index-at

@[d;1 1 1;+;3]
((1 2 3;4 5 6 7);(17 18;19;20 21);(13 14;15 16 17 18;19 20))