I presume they're referring to the (obsolete) Positive/Negative black and white Polaroids, Type 55 (and one of the 3x4 pack films). We'd normally speak of "clearing" the negative from these films after peeling the print off, in order to keep the negative in condition to use for scanning or darkroom printing.

This involves putting the negative, as soon as possible after peeling, into a sodium sulfite solution ("clearing bath") to prevent the pod contents from hardening on the film and remaining fixer from damaging the image.

I've understood that the term is derived from "hypo clearing solution," a sodium sulfite bath used to speed washing of prints (the sulfite promotes the thiosulfate to clear from the print, especially important for fiber based papers). The solution used for Polaroid negatives is much more concentrated, but has the same general effect -- preservation of the negative by promoting removal of fixer (and other chemicals from the pod).