All forms must implement the \Drupal\Core\Form\FormInterface. The interface defines a validation method. The validateForm method is invoked once a form has been submitted and provides a way to validate the data and halt the processing of the data if required. The form state object provides methods for marking specific fields as having the error, providing a user experience tool to alert your users to specify the problem input.
In this recipe, we will be validating the length of the submitted field.
