Split transactions are basically multiple transactions that occurred simultaneously. One example of a split transaction is if you make a sale and accept the payment via a credit card processor who charges a fee.

Split transactions can only be edited in the Edit Transaction window, and cannot be editing directly in the transaction lists.

When editing split transactions, all of the items will go "to" the same account. So in the previous example of making a sale through a credit card processor:
150.00 from Sales Income Account to Bank Account
-5.00 from from Credit Card Processing Fees Expense Account to Bank Account

This example would deposit a net total of $145.00 into your Bank account, while maintaining that your gross income for the transaction was $150.00 and your expenses for the transaction were $5.00.

Each line can be a separate part of the split transaction when editing a transaction. To add a line, simply click on the list of splits below the lowest line.