A content type is a reusable collection of columns and settings that can be applied to a certain category of SharePoint content
In SharePoint, a content type is a reusable collection of settings that you want to apply to a certain category of content. Content types enable you to manage the metadata and behavior of a document, item, or folder in a centralized, reusable way.
Content types provide organizations with a way to manage and organize content consistently across sites, lists and libraries and they also make it possible for a single list or library to contain multiple item types or document types.
You can read more about content types, here.