No LOL .A 'junior barrister 'is a barrister who has not 'taken silk' (become a Queen's Counsel) :) He or she might be of forty years' call ( forty years in practice ) and in their sixties but is still 'junior' counsel.
When you read of a QC and his or her 'junior' that is a reference to the fact that, in most cases (formerly all cases) in court a QC cannot appear alone. The QC is known as the 'leader' and they have to have an assistant, junior counsel at all times in court.That's their 'junior' So if you brief (hire) a QC you always end up with two counsel, the QC and the junior, in court. A QC is also known as a 'silk' because their gown is traditionally made of silk (in practice they usually wear one of some synthetic material, day to day).
Barristers who want to become Queen's Counsel apply to become one. The process by which they appointed is mysterious( though it's not giving any real secrets away to say that enquiries are made of judges and other counsel as to their professional reputation and also about the work they do and the extent and importance of their practice).
A practising QC (there are some who are eminent scholars who never practised) only does work of the highest calibre or gravity.
A trainee , apprentice barrister is called a pupil and the apprenticeship (of twelve months with an experienced barrister, their 'pupil master ') is called 'pupillage'. At the end of their pupillage they hope to become 'tenants' in a 'set of chambers' (offices), so they can practise.