UCAS stands for Universities and Colleges Admissions Service. It's the centralised service that you will use to apply to university.
They also provide information, advice, and admissions services for UK conservatoires (UCAS Conservatoires), for Initial Teacher Training (UCAS Teacher Training), taught postgraduate courses (UCAS Postgraduate), and for young people looking to move to a new school or college after they’ve taken their GCSEs, Highers, or equivalent qualification (UCAS Progress).