Every dollar (or euro, or whatever currency you use) that moves through Rasa Money starts life as a transaction. There are three kinds: an Expense for money going out, an Income for money coming in, and a Transfer for moving money between two of your own accounts (transfers don't get a category, since you're not really "spending" anything). You can type these in yourself, or let them be created automatically — by a recurring subscription, by a debt payment coming due, or by importing a bank statement.
Whatever the source, transactions are the thread that ties the whole app together: they move your account balances, they get sorted into categories, they get matched to your budgets automatically, and they can be linked to a debt if you're paying one off.