As i understand from you i think that f(x) is some kind of function,so that
f(x+a) is obtained by replace every x in f(x) by (x+a)
another expansion about that fuction: f(x+a) is f(x) itself delayed by (a) so it has the same shape of f(x)
note: f(x+a) shift f(x) to the left if a>0...