Mailtrap is a service (free plan available) that you can use to "trap" emails, without actually sending them to their intended recipient. Instead they're shown to you in a user interface.
When working on a development site, it's useful to configure emails to use such a service to prevent accidentally sending test emails to actual customers.
After creating an account, you can find the credentials in the inbox settings.
In MODX, configure the following via System > System Settings:
mail_smtp_passare provided in the inbox settings in mailtrap.