Linux Kernel is an open-source Unix-like operating system kernel. The kernel makes operating system run: it's a monolithic software implementing interaction with hardware on you system and providing interfaces (APIs) for user programs to interact with the same.
Linux Kenel Versions
Current version: Linux Kernel 5.5 (released January 27, 2020)
What is Linux Kernel?
Kernel is a core OS functionality, meaning it looks after all the basic OS functionality like memory and other resources management, supports working with various hardware components via modules and guards secure and performance access to everything running on your system.
Linux Kernel was created in 1991 by Linus Torvalds, since then it grew in popularity and now provides base level for any and every Linux distribution found: Red Hat Linux, Ubuntu, Linux Mint, Debian, CentOS etc.
Any Linux OS distribution is essentially an open-source Linux Kernel with largely similar set of pre-compiled open-source software plus distro-specific customisation.