Spring is one of the most popular enterprise application frameworks in the Java landscape today. It is so popular that it's even considered the de-facto standard for building large scale applications in Java today.
Spring is not just a framework. It's also a complete ecosystem of supporting frameworks and tools that provide you everything you need to build complex applications. It provides support for various common things that most enterprise applications need, like connecting to the database, managing business services, security and so on.
If you are extending code that was written using Spring, or setting out to write a new one, a good knowledge of Spring Core as well as the supporting frameworks used is very important.