Preparing Signal for Sampling
That is not a circuit diagram, but only a block diagram, therefore there is not much we can explain here. 

That block diagram simply states the following:


Some fraction of the mains is "sampled" using a transformer, which is also a safety measure to prevent electrical risks.
The output from the transformer is fed to an amplifier/buffer whose output is low-pass filtered (probably to prevent aliasing in the ADC and reduce noise bandwidth).
The filtered signal is converted using an ADC, whose operation depends on a voltage reference Vref. The voltage reference is buffered to avoid "disturbing" the reference source itself.


Nothing more can be said from that block diagram, except for wild guessing.