Databases are subject-focussed collections of searchable references. They bring together the entries of carefully selected academic journals and provide a platform that allows you to search all of those entries simultaneously. Databases themselves sometimes contain the full-text of an item, but where they do not, UCL's comprehensive journal subscriptions mean you will often have the option to read the full text anyway.
Databases are important tools for ensuring you can systematically and comprehensively survey the literature on a topic area. This is needed for good quality research in any discipline.