A database is an organized collection of data, which is usually stored and accessed from a computer. You actually already have experience using databases in your personal life.
Amazon.com is a database that allows you to sort items you can purchase by categories, such as books, clothes, etc. You can also use filters, such as price, to find the best deals.
Library databases provide you with access to information that isn't available for free online, such as scholarly peer-reviewed journal articles, magazine articles, newspaper articles, and more. The Merced College Library spends $80-90k every year to renew its subscriptions to 70+ databases.
Academic Search Complete is a multi-disciplinary database. It provides access to the full text (complete articles) for more than 4,600 journals, which includes 3,900 peer-reviewed journals.
Many of the the library's databases allow you to sort and filter information sources by source type, publication date, and, in some cases, by language, geography, etc. The filters available will depend on the specific database.