The Contact Changed Workflow Trigger in Workflows allows you to automate actions when changes are made to a Contact Record such as:
Tags being added or removed
DND being activated or removed
Contact being assigned or reassigned to a user
Custom fields being populated or changing to something specific
To add a Contact Changed Trigger, go to "Automations" > "Workflows" > click the "Create a Workflow" button at the top right of the screen.
Then, click "Add New Trigger" and search for the "Contact Changed" trigger.
After the action and rules have been added, click on the save button to save the workflow.