MOS Cascode - Shielding Property
If the M1 drain was connected directly to Vout, then it has the Vout voltage swing on it.
As Vb is constant, the voltage of M2 source is very nearly constant, with orders of magnitude less voltage swing on it than Vout. M1's drain is therefore barely moving at all.
This is important as the major bandwidth limiting effect in a single stage inverting amplifier is the capacitance from drain to gate, and its amplification by the gain of the device.
With M1 driving Vout, the Miller effect times Cgd loads the gate, reducing the bandwidth.
In the cascode, the Miller effect for M1 is reduced to nearly 1. The Miller affect for M2 is not a problem due to the low impedance gate drive.