A saying to denote that one oughtn't have one set of rules
for one party
(usually yourself) and another party (usually someone else) in a similar situation.
For example, the United States is a big fan of bringing the heads of other countries before the war crimes tribunal in the Hague, in spite of the fact that they commit many of the same 'offenses'. Judicious application of this principle would require William Jefferson Clinton to stand in the same dock as Slobodan Milosevic for graphite bomb and depleted uranium usage. Not that that's ever going to happen.
I believe that the original phrase was, "Sauce for the goose, sauce for the gander", or some such.
sleeping_wolf noted that this phrase most often refers to gender-related rules.