A transaction is any change in the balance of your accounts.
It includes all information about how much was spent/received and where the money is from.

Moneydance uses the double-entry method of accounting.
This means that every transaction is a link between accounts or categories.

The value of one account/category will increase and another will decrease when a transaction is made.

You'll find the steps for entering transactions outlined in this article.

Transactions can be between a financial account and an expense or income category.
Or between two financial accounts (such as a transfer from a checking to a savings account).