Bandwidth refers to the maximum amount of data that is sent or received over an internet connection over a specific period.
The internet speed outlined in your contract is the maximum bandwidth at any point in time on your connection. This bandwidth is shared between all users within your household. So if one device has a high percentage of the bandwidth, the other users' speed in the home will be affected.
Bandwidth can also refer to a set of frequency ranges that transmit a signal out. This variety of bandwidth is measured in Hertz.