A party (or counterparty) is a person or company that pays money to you, or to whom you pay money. In the first case, such a party is called the customer. In the second case, the vendor (or seller). Money Manager uses the general term "party".
Creating new party
Click on Parties in the top menu to open the list of parties. Then click on Add… to open the form for creating a new party.
Specify a title and select a default category, which will be automatically selected for a transaction if this party is selected in it. Click the Add party button to save the data.