A trigger is Apex code that executes before or after the following types of operations: insert, update, delete, merge, upsert, and undelete. Developer environments, sandboxes, and trial orgs can create, edit, and delete Apex classes and triggers directly in the Salesforce CRM user interface. Unlimited Edition and Enterprise Edition production orgs can execute and view Apex in the user interface, but modifying Apex in the user interface is not allowed.
Write Apex classes and triggers first in a developer environment, sandbox, or trial org, then tested with appropriate test methods before deploying to production. Install the Marketing Cloud Connect for Marketing Cloud Engagement package in your production org before deploying triggers to ensure that classes are referenced properly.