CAS stands for Confirmation of Acceptance for Studies. A student will need a CAS number to apply for a visa to allow them to study in the UK.
UK immigration rules require the University to sponsor an application for a student visa (this is not the same as financial sponsorship). The University confirms to the UK Government's Home Office that the student has an Unconditional Offer, and that the student has accepted this. The Home Office will then issue a CAS number for the student and send this to the University.
You can read more about CAS numbers on GOV.uk.
A CAS number can be added once the Application reaches "Need CAS" status.