Spring is lightweight, extensible, robust framework (or rather framework of frameworks!) for creating enterprise applications that are easy to maintain.
Spring Core' is the core module that comes with an embedded servlet container (IOC Container) and would enable the core functionalities of spring. You can think of Spring Core as the core engine on which other modules would run.
Spring Core is the starting point for learning spring framework, and having a solid understanding on the core concepts of spring would ease your journey with learning other modules/projects of spring.
This course will cover all required spring core concepts in great detail.