The before plugin, as its name suggests, runs before the observed method.

When writing a before plugin, there are a few key points to remember:

1. The before keyword is appended to the observed instance method. If the observed method is called getSomeValue, then the plugin method is called beforeGetSomeValue.
2. The first parameter of the before plugin method is always the observed instance type, often abbreviated as $subject or directly by the class type – which is $processor in our example. We can typecast it for greater readability.
3. All other parameters of the plugin method must match the parameters of the observed method.
4. The plugin method must return an array with the same type and number of parameters as the observed method's input parameters.

Let's take a look at one of Magento's before plugin implementations, the one specified in the <MAGENTO_DIR>module-payment/etc/frontend/di.xml file:

<type name="MagentoCheckoutBlockCheckoutLayoutProcessor">
    <plugin name="ProcessPaymentConfiguration" 
            type="MagentoPaymentPluginPaymentConfigurationProcess"/>
</type>

The original method this plugin is targeting is the process method of the MagentoCheckoutBlockCheckoutLayoutProcessor class:

public function process($jsLayout) {
    // The rest of the code...
    return $jsLayout;
}

The implementation of the before plugin is provided via the beforeProcess method of the MagentoPaymentPluginPaymentConfigurationProcess class, as per the following partial example:

public function beforeProcess(
    MagentoCheckoutBlockCheckoutLayoutProcessor $processor, 
    $jsLayout) {
    // The rest of the code...
    return [$jsLayout];
}