This is the first question which should come to your mind when you start reading and knowing Linux because without knowing what a kernel is you can not move ahead.
Let me help you understand kernel
What is Linux Kernel?
It acts as an interpreter between Linux OS and its hardware. It is the fundamental component of Linux OS and contains hardware drivers for the devices installed on the system. The kernel is a part of the system which loads first and it stays on the memory.
What is monolithic kernel?
A monolithic kernel is set up so that all the drivers are compiled in the kernel. There's no need for the user to load any modules. However when you install new drivers, you need to recompile the kernel. As a result kernel becomes larger and slower ans requires more memory.
What is modular kernel?
A modular kernel is set up so that most hardware drivers are compiled as modules. As a result you don't have to recompile the kernel when installing new hardware devices.
In practice kernels are combination of monolithic and modular kernels where some drivers are compiled into the kernel and some are compiled as modules.
Some bullet points
The Linux kernel acts as a mediator between system's hardware and software.
It is also responsible for memory management and multiplexing
It divides a CPUs processing capacity between application so that they can run simultaneously