[7 [b c]] is a "function" that, when applied to a, firstly applies b to a then applies c to the result. It is therefore just function composition.
Here is a reduction:
*[a [7 [b c]]] =>
*[a [2 [b [1 c]]]] =>
*[*[a b] *[a [1 c]]] =>
*[*[a b] c]