*Offering support* is offering support. Affirming means to strongly agree with the patient if they say they have concerns they might be transgender, sometimes to the points of actively pushing them in that direction, instead of offering support as they figure it out themselves.
If the patient leans towards deciding they're *not* trans, the therapist should also support them in *that* direction, but that's not what's happening these days, because even some therapists with ethics are still afraid that if they don't actively affirm transgenderism, they may be accused of transphobia, and risk losing their practice, and even their license.