SKSE (Skyrim Script Extender) is a component which most (if not all) complex mods depend on. This is very easy to install, it only takes 2-3 minutes. Head over to the SKSE website and download the appropriate version for your game (it’s important that you do not make a mistake here).
You will also need a utility like 7zip to unpack it anywhere on your machine.
When unpacked, simply copy all the files into your game’s root folder and every time you update it, do the same and overwrite the old ones. It’s important to keep this updated or mods won’t work, unless you don’t update them either and just stay where you are in terms of runtime versions.
I suggest using a robust mod manager like Mod Organizer 2.
If you’re not familiar with this, take a look at my how to start modding Skyrim guide. I explain in more details there.
To test if you’ve installed SKSE, open the console and type getskseversion (this will return the runtime value if all is well).