Spring is an application development framework, that provides the "plumbing" between various services. It typically provides a way of configuring applications in a flexible way and tries its best to be technology agnostic.
Spring initializr is a tool used to initialise Spring boot applications. Spring boot is an extension to the spring framework, which provides the default configurations for various services.
If none of this makes any sense to you, then just learn the basics of core spring; java config / component scanning. Then look into Spring Boot.