It is a cylindrical variable capacitor, there to provide AC compensation for the input with increasing frequency. It forms a potential divider with the variable capacitor and the input capacitance of the meter input circuit, and is adjusted so that the ratio of these capacitances is the same as the ratio for DC voltage. Thus the response of the meter is constant for DC and Ac voltage. This is more important at higher frequencies like over 1kHz, where the capacitances will pass enough reactive current to have a noticeable effect on the voltage at the actual ADC input of the meter.
Cylindrical so that the outer electrode ( connected to the input) provides shielding to the inner more sensitive electrode so hand movement near the meter will not affect the reading, as this small capacitor is the most easily affected by nearby conductors. If you trace it you will find it is attached across the input sampling resistor network ( the blue resistor array to the right of it in the last picture) and will be on the first and second pins, the ones with the 13M resistor inside, and the ones with the large creepage space. The capacitor has to be rated for operation at more than the meter is rated for, as it cannot break down in service without destroying the meter.