Matriculation is the ceremony which marks your formal admission to the University, usually at the end of your first week. Normally this takes place in a ceremony at the Sheldonian theatre at which academic dress is worn.
Visiting and Recognised students are not required to matriculate. For a full list of courses that do and do not have to matriculate, see the University web pages referenced below.
If you are a graduate who completed undergraduate study at Oxford, you do not need to matriculate again.
The Academic Office provides information about your matriculation in the Information for Undergraduates / Graduates pack that you will have received before arrival.