It stood for logical AND, so really your search term would be read as:

foo AND bar AND baz. It would be more accurate to type it as foo + bar + baz.

They've unfortunately conflated "must have" and "spelled exactly", which aren't the same thing.

This explains so much. I thought they were distinct operators. I thought quoting meant must match exactly, and the plus meant must be present. So +"baz" meant it must be exactly baz, and it must be present. +baz meant baz, or some variant like bazzes must be present.