Bandwidth and transfer are two different things. Bandwidth is how much data you can transfer in a period of time. For instance, a 10BaseT ethernet connection has a bandwidth of 10Mbps (megabits/s), which is about 1.25MB/s (megabytes/s).
If someone is offering you a certain amount of transfer each month, they're telling you how much data they'll allow you to transfer in a month. Many times, the terms bandwidth and transfer are confused, but you can tell what they are talking about by the context in which the terms are used.