It is the bandwidth of the Y-amplifier.
At that frequency the amplitude displayed drops by 3dB. Say you have a signal of 1Vpp. If the frequency is equal to the bandwidth, then the scope will display the signal as having only 0.707Vpp.
So it's a measure of how "good" the scope is. For home projects even a 10MHz scope can be extremely useful. I personally have a 25Mhz one.