Validate a change set before deploy
Key Takeaway
Validate an inbound change set to find out ahead of time whether its deployment will succeed. Salesforce rolls the validation back, so the org doesn't change.
A validation is a test deployment of an inbound change set, used only to check results. Salesforce runs the deployment and shows you the same success or failure messages an actual deployment would give. It then rolls everything back, so no components are saved and the org isn't changed in any way. A successful validation can also make the later production deployment faster through quick deployment.
When to validate
Validation is most useful when you plan a deployment on a schedule, such as during off-peak hours, and want to know ahead of time whether it will succeed. You don't need to validate before every deployment, because validation takes time and the org is locked while it runs.