The amount of data that can be transferred through a connection in a given time interval, normally specified in megabits per second or mbps. 10 mbps means that 10 megabits can be transferred in 1 second. 8 bits eqals 1 byte, so 10 mbps is approximately 1.25 megabytes per second or 324 gigabytes per month.
That does not mean that a dedicated 10 mpbs line will fulfill your needs if you need 300 gigabytes of data transfer per month. Traffic to websites are not constant but vary through the day and week and if your visitors in total load 10 mbps in a given moment, the 100% capacity usage will result in slow data transfer for some/all of your visitors. So if you are shopping for a dedicated line, dimension it for peak hours and then some.