The year at which the barrister is formally recognised to have passed the vocational component of training and has been admitted to the Bar (being “at the Bar” is the term used to describe the profession of being a barrister).

The year of call is commonly used to describe a barrister’s seniority.  So, if a barrister is 3 years call that is very junior and if 20 years call, i.e. called to the bar 20 years ago, that is a senior barrister.