AD627 question about the REFERENCE INPUT , what is this means？
This is the input impedance seen at the AD627's reference input given that there's no gain-setting resistor connected on the RG pins. You want to avoid adding resistance to this reference terminal as it will introduce voltage error. It's also mentioned on the datasheet that this REF pin should be tied to a low impedance input.
Looking at the AD627's datasheet, the theory of operation explained in detail the transfer function given that there's no gain-setting resistor present. It explains how the matching and precision of these resistors ensures low error of the part.
Hope this helps roy.wang.