When you first set up your Zap trigger, Zapier will attempt to find some test data from your trigger app to use in the Zap. For example, if your trigger is a new email in Gmail, Zapier will pull in a recent email you've received. This email can then be used as test data in an action step later in the Zap, such as to customize a Slack message with data from the email.
As you continue to set up the rest of your Zap, you may realize that you need different test data from your trigger to complete the rest of your Zap or make it easier to set up certain Zap actions.