Its a binary that calculates how many lines of code you have written in a project. Its git "aware" and like "okay".
Basically it excludes all files excluded in your .gitignore like target and such so they don't artificially inflate your numbers.
Additionally it excludes some none source code like files like json and toml.
Run : okay-loc --help for up to date help.
OR you can try running okay-loc -p ./ to scan your current directory.