You may be asked to attend a credibility interview as part of the application process. Interviews are normally carried out by Skype, although sometimes they will consist of a face-to-face interview.
Your interview will normally be held at Paddington in London. During this interview a Home Office member of staff (either in person or by Skype) will assess whether you meet the English language requirements, and also if you are a 'genuine student.' If you are asked to attend such an interview you must do so (unless you have a good reason as to why you cannot), otherwise your application is likely to be refused.
On receipt of your interview letter, please make sure that you complete the 2 page confirmation letter and email it back. Also, don't forget to book an appointment to see one of the International Student Advisors.