Marking lead as won will mean that you have successfully clinched the deal. A lead can be marked as won from any stage in the pipeline.

To mark a lead as won click on the lead in the pipeline.

Click on the option Mark as won

Once you click on Mark as won, the lead will be moved from the pipeline to Archived Leads
All the information pertaining to the lead such as notes, todo's, files etc will be archived along with the lead.
The lead can now be viewed in the archived lead page.