Deleting a workflow removes it from your library. It also removes all files connected to that workflow including files uploaded to components (such as spreadsheets and document templates) as well as generated and uploaded files. You might want to consider downloading all of the files in the file library, exporting any form data and workflow data etc.
Once a workflow has been deleted, it cannot be recovered so be sure you want to delete it.
To keep a copy of the workflow in case you want to use it again in the future, you can deploy it to your private marketplace before deleting the project.