Databases allow you to access articles and other resources from many journals and publications at the same time. Unlike a Google search, it connects you directly to what your library has, and has a better chance of relevancy. It also allows you to filter your results to help you find what you need. In addition to peer-reviewed journals and articles, databases have many useful resources, like newspaper articles, topic reviews, videos, ebooks, and much more.
Remember: databases have more resources than peer-reviewed articles, so just because you get something from a database doesn't necessarily mean it's peer-reviewed. If your professor asks for peer-reviewed sources, always check!