This is done by using the Placeholders in the Playbook Actions. Placeholders are expressions that reference fields on events, alerts, cases entities and other elements. Placeholders will hold a place for the data that will be extracted when the action actually runs.
To use a placeholder, click on the “< >” button inside a text field (e.g. the message field in a “Send Email” action) and select the content you would like to extract when the action runs.

You can string several placeholders in a row to create richer content.
Notice that a placeholder will always begin and end with a square bracket. Inside those brackets will always be the path of the required content. For example: The [Alert.Name] placeholder will check the alert name and put in the message before sending the email.
Remember that Placeholders can be used in email templates, allowing you to set a template once, with all the required placeholders and use it when needed (Setting > Environments > Email Templates).