Double-entry bookkeeping records every transaction twice: once for where the value went, and once for where it came from. Those two sides must always be equal, and that equality is what proves nothing was lost, invented, or entered in one place and forgotten in another.
It is roughly six hundred years old, it is what every accounting system worth trusting uses, and it is the reason a Balance Sheet balances at all.
The idea in one example
You sell a $100 item on a marketplace.
Single-entry bookkeeping — a spreadsheet — records one line: +$100 sales. That is all it knows.
Double-entry records two:
| Account | Debit | Credit |
|---|---|---|
| Marketplace Receivable (the money owed to you) | $100 | |
| Sales | $100 |
Read it as a sentence: the marketplace now owes me $100, because I made a $100 sale. One side says what you now have, the other says why you have it. Both are true, and they are the same size.
When the marketplace takes a $13 fee:
| Account | Debit | Credit |
|---|---|---|
| Marketplace Fees (an expense) | $13 | |
| Marketplace Receivable | $13 |
I incurred a $13 expense, and it came out of what the marketplace owed me.
Why the spreadsheet version fails
The single-entry version records $100 of sales and, separately, $13 of fees. It never records that the two are connected, or that what you are actually owed is $87.
So it cannot answer: how much is the marketplace holding right now? Is that number right? Did every fee I was charged make it into my records?
The double-entry version answers all three, because the amount owed to you is an account with its own running balance — one you can compare against what the marketplace says it is holding. That comparison is the whole point. It is a check that either passes or fails, and a spreadsheet has nothing to check against.
What "the books balance" actually means
Add every debit in your entire ledger. Add every credit. They are equal — not approximately, exactly.
That is not a formatting nicety. If they ever disagree, something was recorded wrong, and the disagreement is the alarm. A system that cannot go out of balance is a system that cannot tell you when it is wrong.
This is also why an entry can never be "just one number." There is no such thing as money appearing from nowhere: it always came from somewhere, and both ends get written down.
What this means for you in practice
Mostly, that you do not have to think about it. SalesImporter writes both sides of every marketplace transaction for you, and the reports come out of those entries rather than being tallied separately.
Where it becomes visible is the moment something does not add up — because with double-entry, there is a specific account with a specific balance you can go and check, instead of a total you can only squint at.