This reference guide provides detailed information about the Spring Framework. It provides comprehensive documentation for all features, as well as some background about the underlying concepts (such as "Dependency Injection") that Spring has embraced.
If you are just getting started with Spring, you may want to begin with the lighter "Getting Started" guides that are available from https://spring.io. As well as being easier to digest, these guide are very task focused. They also cover other projects from the Spring portfolio that you might want to consider when solving a particular problem.
Getting Started Building a RESTful Web Service would be an excellent first choice to get your feet wet.