When we have a doubt (safek) in a dioraissa, we must act strictly. For example, if you can't remember if you said kriyas shma, you must repeat it. However, for a safek dirabanan we act leniently. For example, brachos are midirabana (though Ramban holds Birchas Hatorah is midioraissa) so if you forgot if you made a bracha, or if it is uncertqain if you need to make a bracha in a certain case, you do not make the bracha. That which we act strictly for a safeik dioraissa, is that itself midioraissa, or do we only treat it lechumra midirabanan? Rashba says it is midioraissa, but that Rambam holds it's only midabanan.
The Rashba says that Rambam's source is this Gemara: A safek mamzer is midioraissa allowed to enter the khal, but midirabana is not permitted. Rambam cites this in Issurei Biah 15:21. Rashba assumes that because safeik mamzer is mutar, all safek midioraissa cases are mutar midoraissa. Rashba argues and feels that Safek mamzer is an exception, and that's why there is a gzeiras hakasuv to allow safek mamzer in the khal. (The teshuvas Harambam that Rashba cites is nebulous - I am not aware that we have such a teshuva by the Rambam.)
Rambam in Tumas Mes 9:12 also discusses safek dioraissa lekula. Raavad asks that normally we say safeik dioraissa lechumra [like Rashba]. See Kesef Mishna there (who says Raavad seems to contradict himself in Kilayim 10:27), quoting from Ran Kiddushin 15b in Rif pages.
See Shev Shmaitsa Shaar 1 for more discussion about safeik dioraissa.