Why An op-amp's open-loop gain is huge at DC but rolls off at 20 dB per decade above its dominant pole, so the product of gain and bandwidth stays roughly constant. That constant is the gain-bandwidth product, and it is numerically the frequency where the open-loop gain has fallen to unity (0 dB). Knowing it lets you predict usable bandwidth: a stage set for a closed-loop gain of 10 built from a 1 MHz GBW device works to about 100 kHz.
Watch out The idea of a maximum filter frequency is tempting because GBW does limit filter designs, but the specification itself is a property of the amplifier alone, not of any particular filter circuit built with it.
GBW = the unity-gain frequency. Gain times bandwidth is constant, so gain 1 means bandwidth is the whole GBW number.