Here's a simple map: a=1, b=0, so f(x) = x(31). That means you would encode the word "math" to m->m, a->a, t->t, h->h, or in other words your coded word is "math". Clearly this isn't a good way to hide information.
Let's try a more complicated map: a=1, b=1, so f(x) = x+1(31). Now "math" -> "nbui". This is better in that someone reading the word "nbui" wouldn't immediately recognize this as the word "math.
Can you keep going?