Transfer is one of the system categories. A transfer record is used for transferring money between user's accounts. E.g. by withdrawing money from an ATM (Bank account > Cash).
Every transfer creates 2 new records within the same category "Transfer": one as an income and one as an expense. Each for a different account.
Transfer visibility can be disabled under filter settings.
How to create a transfer?
Use the small FAB button (+) on Dashboard, and then choose the Transfer option. Or, create a transfer by clicking on the small arrow icon between the Account and Category boxes on the first page of the New Record form.