Troubleshooting Flow Errors Using the Debug Button in Flow Builder | Salesforce Platform

As a developer you should test all customizations before rolling them out to users, and flows are no different.The Debug button on Flow Builder lets you fine-tune the flow’s behavior, identify, and fix bugs.It lets you enter values for the flow’s input variables and display debug details while running the flow.
For more information you can check this article
https://help.salesforce.com/articleView?id=flow_test_debug.htm&type=5
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