A content database is a database file that stores content for one or more site collections for SharePoint web application. The content can be pages,files,documents,images and much more. So if the Site Collection has more number of SharePoint sites, the content database size grows rapidly.
One SharePoint web application can have more than one content database mapped to it and the max limit is 300 ContentDB's per Web Application. If you want to add more content databases to web application, Go to Central Admin -> Application Management -> Manage Content Databases -> select web application -> Add a content database.
Maximum Content database size is 200 GB in general usage scenarios.