Merchants can be used on Expenses & Subscriptions. They help separate the data so you can see how much your expenses are for going out ETC...
The first step to create a merchant is to go to the Merchants on the Dashboard:
This will be the same on mobile as well. You will need to click on the create merchant button:
You will fill the Name & About fields in then click the Save button.
You have now created a merchant to be used in expense or subscription. You can now see the merchant in the Merchants home.
You can now click on your merchant to view it.