Need help with schematic, not understanding

What is that switch between AC to DC mean ? What kind of part is it ?

please publish a full schematic so that comments could be more realistic

With the switch in the DC position there will be an offset voltage at the output of the INA321. In the AC position the offset voltage is 0 volt.

The DC position couples both the AC and DC portion of the signal, it's DC coupled. In the AC position only the AC portion is coupled. It connects a feedback integrator which acts the same as if there were a high-pass filter coupling capacitor in series with the output (there's a notation on the schematic which tells you that the HPF -3dB point is 0.5Hz). The advantage of the integrator is that you can use a high resistor to generate a low-frequency rolloff, which may not be practical with the usual LPF in series with the output.