Socially mediated is a term used in learning that simply means that reinforcement is gained through the efforts of someone else.
In this clip, Joey and Chandler’s access to food is socially mediated. They are relying on someone else to provide them with the great stuff that they want.
Not all reinforcement is socially mediated, at least for people who have some skill set that allows them to gain access to things independently. As early as age four or five, you can get up and get your own drink from the refrigerator, which is direct reinforcement. No one else’s effort is involved in your access to that reinforcement—unless you consider that the four year olds’ parents had to stock the fridge. Some reinforcement is accessible without having to involve anyone else. Socially mediated reinforcement involves someone else to gain access to it. This is especially true of infants, who have no biological ability to access reinforcement without the help of someone else.