[orthoxerox]

In a nutshell, double-entry bookkeeping is tracking all your money in two ways:

- where has it come from/has it forever gone? - where is it now?

So, you start a simple ledger of having $100 in cash with a transaction like this:

    Dr "cash" Cr "original funds" $100

Then you spend some of it on food and loan some to Bob:

    Dr "food expenses" Cr "cash" $25
    Dr "loan to Bob" Cr "cash" $20

Bob pays you back $22:

   Dr "cash" Cr "loan to Bob" $20
   Dr "cash" Cr "interest income" $2

You can't write 'Cr "Bob" $22', because... I don't want to get into the principles of accounting, but basically all asset accounts only go one way. You can't have minus two dollars in your pocket, and Bob can't owe you minus two dollars either.

Some of the accounts, like "original funds", aren't very useful by themselves, but they are the only way to make sure "money I literally have in my account/pocket", "money I owe people" and "money that people owe me" can all be counted together: if you tally up both kinds of the accounts, the total sum should be the same, just with the opposite "sign".