When you work on the different leads the lead can be marked as won or lost and every leads has its own reason to keep the record of the reasons of the lead you can add reasons with the particular lead.
To create the reason you can click on the More button, then click on the Reason option. Click on the +Add New button.
Fill out your reason and select type option with the drop down choose the reason for won or lost, then click on Add button. By this you can add your reasons to the lead.