If you have a prepaid account it means that you have to buy credits to load onto your account before you can send messages, i.e. you need a positive account balance. When your balance is depleted, you won't be able to send more messages until you've topped up your account. Prepaid is mainly for smaller clients, i.e. SMEs.
If you have a postpaid account, you can send an unrestricted number of messages. Your account balance is allowed to be negative, and an invoice is calculated at the end of the billing period based on your actual usage. Postpaid is only available to larger clients, i.e. Enterprises. To be eligible for postpaid invoicing, you need to be generating roughly $3,000 in monthly revenue.