Synchronize Customization Sandbox with Main Sandbox
If your sandbox contains conflicts with the main sandbox, and your sandbox was created before a merge in the main sandbox, you must resolve these conflicts before you apply and merge your changes into the main sandbox.
Validate your sandbox to identify and resolve errors, and then synchronize your sandbox with the main sandbox's latest changes to prevent inadvertently overwriting another developer's work.
This practice is also useful if another developer merged a new object into Main and you need to include that new object in your own customization.