require is a function. .main is a property on that function so you can reference require.main. That part of the doc you are referring to says that you can write code like this:
if (require.main === module) {
// this module was run directly from the command line as in node xxx.js
} else {
// this module was not run directly from the command line and probably loaded by something else
}
module in that above code is a variable that is passed to all modules that are loaded by node.js so that code basically says that if require.main is the current module, then the current module is what was loaded from the command line.
The code for setting that property is here: https://github.com/nodejs/node/blob/master/lib/internal/modules/cjs/helpers.js#L44.