Cogender is an inclusive gender identity that is the union of two or more gender identities. A cogender person is okay being identified as any of the genders included in their identity. You can be cogender and not know it at first if your gender happens to include the gender you were assigned at birth. The difference between a cogender person whose gender happens to include the gender they were assigned at birth and a cisgender person is that the cogender would okay with being identified as another gender.
One format of expressing this identify is X coY where X and Y are genders. Example: girl coboy.
A cogender person may want parts they don't have that are associated with their cogenders, but they don't need them. This doesn't mean they don't experience dysphoria, just that they are fine with the parts included in their gender identity. Example: an agender cogirl person assigned male at birth may feel dysphoria over their genitals (and the reactions thereof) and may want breasts but doesn't need breasts.
One type of cogender identity is cofluid, which is a genderfluid version of it. In a cofluid identity, one part of the cogender identity is static but the other part(s) change, possibly to match the first part. For example: a cofluidboy has a cogender identity that always includes male, but the other identities vary.