Bandwidth has several related meanings, for our purposes in the wireless industry there are two primary definitions:

Bandwidth (computer)digital bandwidth: a rate of data transfer, throughput or bit rate, measured in bits per second (bps).

Bandwidth (signal processing) or analog bandwidth, frequency bandwidth or radio bandwidth: a measure of the width of a range of frequencies, measured in hertz.

Bandwidth is a measure of available or consumed data communication resources.  As technology grows and our desire for seamless connectivity grows, bandwidth use increases as well.