What's meaning of cd <v8 checkout> ?

[zhangzhuang15]

I install your plugin, but find that I still do installation of language server. I don't understand what's meaning of cd <v8 checkout>, what is v8 checkout ? And invoke these commands in v8 source code root directory?

[Liedtke]

And invoke these commands in v8 source code root directory?

That's exactly the meaning. cd into the directory where your v8 checkout (git repository) is located.
Then at that location you can run the autoninja command from the README.
If you don't have a build directory yet for the autoninja command you'd have to create it using gn gen as described by the chromium build instructions (link).