score:5
Does this mean that I have 15 days to use the SIM card before it expires?
It means the SIM card will expire 15 days after it is activated.
Once it expires, one can buy some x-day (1 ≤ x ≤ 30) additional package using the same SIM card, e.g. via https://topping.truemoveh.com/ for TrueMoveH SIM Cards. Typically one "tops up" (= adds) the money by going to a 7-eleven (they'll ask for the SIM card phone number + cash: no credit card accepted for tops-up, I don't know why), then go on https://topping.truemoveh.com (mirror) to select which package to purchase (in this example, 1k THB were topped-up at a 7-eleven):
Or one can buy top-ups in some stores: