This refers to one of the op-amp inputs when the other input is grounded (when the op-amp is in a feedback configuration). By the ideal op-amp rules, the op-amp does whatever it has to do at the output to make the input voltages equal. So if one op-amp input is at ground, the other must be too, even if it's not physically grounded. (Of course this only works if the op-amp is in operation.)