It's correct what he said. This is how it works, write this down (i'm going to use my account as an example):
I have a 5GB account with WebAfrica. Let's say I, for some reason, have only 1GB left in the middle of the month, I can instantly order more "prepaid bandwidth" at R75 per GB on the WebAfrica homepage. Then, after my remaining 1GB is used up on my "normal" bandwidth it will start to use my prepaid bandwidth that I bought. Let's say I bought an additional 2GB of bandwidth.
Now the end of the month comes in but I only used 1GB of my 2GB prepaid that I bought. NO WORRIES. It then rolls over to the new month and instead of my normal 5GB, I now have 6GB (5GB+1GB prepaid)
It also doesn't start using your prepaid, it uses your normal bandwidth first and only when that's used up does it start to use your prepaid bandwidth again.
In closure...and awesome idea would be to open an account and then buy about 2 or 3GB of extra prepaid bandwidth to act as a "buffer". If you don't use it, fine. But at least it's always there if you need it
Note: ONLY the prepaid credit rolls over month after month. Your normal bandwidth from your 3 or 5GB account always resets and does NOT roll over.