I agree Neko. Not every attribute or ability you earn is something you get when you're born, by birth. In this case this is called 'psychic development'. Basically, even with a gift you will not be fully developed in a spiritual and psychic way. Even as a gifted and psychic person you will need to practice and develop your psychic abilities. Sometimes people are 'more gifted' and sometimes they're 'less gifted', but yet, people from both of the categories must take care and practice in order to psychic develop themselves and actually get somewhere using their psychic abilities and gifts they got.
People might be confused here because they do not really know the definition of the word 'ability' or mediumship.
The definition of the word exactly is : power or capacity to do or act physically, mentally, legally, morally, financially, etc. competence in an activity or occupation because of one's skill, training , or other qualification. (from dictionary.com).
I underlined the word training because mostly the misunderstanding occurs because people think that the term ability means something which is a gift or a skill you got by birth, while you can earn these skills and get this ability developed and found within yourself by the training, even if one has less potential than another. The word ability as very misunderstood. So this is why I agree with Neko.
As for mediumship itself. To make it short, a medium is someone who can connect to people, either spiritually or physically and channel the energy. A medium is a living conduit. They allow spirits to take them over in a controlled environment and can read physical people being able to empathise with them, being able to kind of 'share' their energy and personal experiences them in the moment they do their reading, picking up on how they feel, think, desire, etc performing this.
As all arts and practices, this can be achieved by training as well. Doing the actions described upon are all achieved by training, either by 'gifted' people who might not even be aware of the gifts they have, and either by people who need to practice and train themselves in order to develop their skills a bit more hardly.
You may read and learn about psychic development and mediumship to get a general idea about this subject.