Kexts stands for Kernel Extensions, Kernel is actually the heart of macOS, It is basically the one that gets your Operating System boot & work. The function of Kexts are similar to Drivers in Windows. Kexts extend the functionality of macOS by providing additional codes to the kernel. Basically, Kexts are loaded when macOS boots. Simply Kexts are similar to Drivers in Windows, Different hardware needs different drivers that are the basics.
macOS comes with hundreds of defaults kexts, When you use a Macintosh you will probably never have to install any kexts because every Kexts which are needed by your computer are already bundled with macOS.
System kexts are usually found in /System/Library/Extensions folder (Shortly /S/L/E) but most third-party kexts can be found in /Library/Extensions folder (Shortly /L/E). Users were allowed to install kexts to /S/L/E folder in OS X Yosemite and previous Mac OS X Versions but starting from OS X EL Capitan Apple restricts write access to many system folders including /System/Library/Extensions folder. Users can install Kexts /S/L/E by disabling System Integrity Protection or Kexts can be installed to /Library/Extensions folder.