Every posted transaction, not just a summary. An invoice, a bill payment, a journal entry, a bank deposit; anything that touched an account shows up here as its own line, with the debit or credit amount and which account it hit.
Grouped by account, then by date. Unlike a bank register, which shows one account's activity, the general ledger walks through every account on your chart of accounts in order, showing that account's full activity before moving to the next one.
How to run it. Go to Reports, search for General Ledger, and set the date range you need. You can filter it down to specific accounts, a customer, a vendor, or a class if you don't need the entire company's activity at once.
What 'split' means when you see it on a line. A transaction that posted to more than one account at once, a bill split across two expense categories, for instance, shows as Split on this report rather than a single clean account name, since there isn't one single account to name.
Why it's the first stop for finding an error. If a report elsewhere looks wrong, the general ledger is where you trace a number back to the individual transactions that produced it, rather than guessing at what might be off from the summary alone.