The validate function allows you to have your diagram validated based on the BPMN guidelines. A window on the bottom will appear after the validation is finished which will tell you if your diagram is correct, or if it needs to be fixed and what changes should you implement.
Navigate to the "BPMN Diagram" panel and click on the Validate button.
If your diagram is correct, the log will state that no issues were found.
If your diagram has errors, the Log Viewer will state which errors you have and give you suggestion on where to find the study material to fix the errors. You can also click on the blub for a more detailed description of the error.