CAS stands for Confirmation of Acceptance for Studies.
According to the UK immigration rules, the University has to sponsor your application for a student visa (this is not the same as financial sponsorship). For this, you need a CAS number to apply for a student visa, which allows you to study in the UK. After receiving the admission letter, students need to confirm their acceptance at the preferred university as per the timelines mentioned by the university.
Some universities may ask for tuition fee/admission confirmation fees. They will also request students to show the financial documents that will cover their tuition fees. Once the student confirms their admission, universities will release the CAS letter. After receiving the CAS letter, students can apply for their respective student visas.