Glad you asked! A kext is a kernel extension that adds functionality and hardware support to macOS. There are some rules around kexts that you need to know, and here they are.
/System/Library/Extensions (/S/L/E) - This is where Apple stores the kexts that macOS uses to boot and operate. If something is telling you to put a kext here, hard stop because it's probably wrong.
/Library/Extensions (/L/E) - This is a place for 3rd party developers to place signed kexts for hardware that you buy and connect to your computer. These developers have been granted a kernel signing certificate that is used to sign the kexts which tells macOS they're safe to use. Many (if not all) kexts that we are using are neither signed or endorsed by Apple. If something tells you to put a kext here, hard stop as the same rule applies.
EFI/OC/Kexts (OC/K) - This is the location that OpenCore uses to inject functionality into the operating system for your Hackintosh to boot and operate. This is where all of your Hackintosh kexts go. Did you notice the lack of a slash before OpenCore? That's because you need to reference where you've mounted your EFI partition before adding OC/K. Remember that, you'll need to add the mount path anywhere in the guide that it's mentioned.
Following this policy keeps your system clean and allows you to fully enable System Integrity Protection increasing the security of your laptop.