Evidental Mediumship readings are basically sessions where there are messages that give evidence that the medium is speaking with your loved one and their spirit is in the room. There could be physical traits, visions of memories, or even descriptions of odd quirks of their personality.
The intention of Evidential Mediumship is for you to become comfortable with the fact that, though skeptical, it is plausible that your loved one is with you which provides a basis of trust with the medium so you may have a deeper communication with the spirit.
Once trust is established, you are able to communicate and ask questions of the spirit and be able to have an open conversation with the knowledge that you are truly with them and only using the medium as a conduit for the session.