Basics

The main file in plugin (which is index.js or it is specified in package.json main property) is required to export either installer/initializer function or object with it.

// Both variants are correct
module.exports = core => {
    // ...
};

module.exports = {
    install (core) {
        // ...
    }
};

The argument of installer is plugin context, which is a special object that lets you registering plugin's functionality. Here is an example of registering database driver below:

class MyCoolDB {
    // Full example will be available later
}

module.exports = core => {
    core.db(MyCoolDB, 'mycooldb');
};

All available types that can be registered are described in this topic "API / Plugin context".