What's a concurrent access right?

Quite simply, the number of concurrent access rights given to group of users sets the number in that group who can access either the server or applications at any one time (depending how the rights are assigned)

A different way to think about it is to imagine each user has a "key" to a house (the server), but that the house only has a limited number of "doorways" (a concurrent access right). Once all the the doorways are in use no-one else can enter, until a doorway is freed up (in our analogy when someone logs off the server).