The Kernel is the core part of any operating system, including Linux. It's the fundamental layer between your hardware (like your CPU, memory, and GPU) and the software you interact with (like programs and applications). When people refer to Linux, they usually refer to a combination of the Linux Kernel with other important Software (Like GNU Utilities) to make a fully functional Operating system to use.
The Linux Kernel itself is a Modular and Monolithic Kernel, meaning that it incorporates most of the Core functions within the Kernel itself (Like Process management, device drivers, Memory management, File systems, Etc). These functions can be modified to be loaded or unloaded dynamically and even replaced with alternative modules. This modularity allows for easier customization to the kernel while also providing extended functionality as needed.