EMPLOYEE0Rounding is the billing increment of a product. For example a 4 minute rounding means that all your calls will be billed in increments of 4: if you speak for 01:30 the charged duration will be of 4 minutes. A 1 minute rounding would mean that the same call of 01:30 would be rounded up to the next minute, so the total charge would be for 2 minutes.
A higher rounding usually goes together with a low rate. These products are for heavy users (very frequent long calls- over one hour each). A lower rounding gives you more control over your charges, but it also comes with a higher rate (intended for a more moderate usage).