Explanation:
A shows the initial value or y intercept.
Why.
A exponential function is shown as

If we are trying to find the y intercept, we first set this equal to y

Set x=0

And solve for y.
Remeber that anything raised to the zero power is 1. So

So we have

Anything times 1 is itself so we have

This means that the y intercept of a exponential function is (0,a). A gives us the y intercept.