If the GBWP ( Gain Bandwidth Product ) of an operational amplifier is 1 MHz, it means that the gain of the device falls to unity at 1 MHz. Hence, when the device is wired for unity gain, it will work up to 1 MHz (GBWP = gain × bandwidth, therefore if BW = 1 MHz, then gain = 1) without excessively distorting the signal.
Bandwidth is the frequency range. High means a large range of frequency lower means less. When the gain falls below one it is no longer able to amplify.