A base plan is what keeps your account active and is renewed every 30 days. A base plan, at the very least is required to have a prepaid account.
Boosters/Top ups are extras that you buy in addition to your base plan but are bought at the time of purchase, (not put on your monthly bill). These are for when you need more than what your base plan offers.
The $15 base plan only includes texting but you can buy a talk booster if you would like to make calls.