This comes back to a system of accounting called "double entry bookkeeping". Basically, whenever you have a transaction, it should affect at least 2 separate accounts e.g. a loan gets you $10k in cash, but adds $10k to your liabilities (debt) so overall, the loan hasn't changed your financial position because you'll have to pay it back later. This allows for a cross check to make sure you haven't missed anything in your accounts.

If you do a job on account (get paid later) you get income so you add this to your "sales" account as a debit (increase in assets) but I also need to show where this money is going so I credit my wages and inventory accounts to show that I no-longer have the materials I used and I have to pay my people. I also need to credit my profit account for the left over money. Now my debit (the sales revenue) balances the credit (what happened to that money). The money owed to me counts for accounting purposes as an asset equivalent to having the cash though we also need to track cash since I can't transfer that debt to my suppliers to pay my bills

Later, I get paid for the job so now my "sales" account is credited (reduced) because I am no-longer owed money but my "cash" account is debited (increased) as I now have the money in my hand / bank account. Again, my books still balance - since this was just collecting money owed, my overall position hasn't changed.

Balancing the books means going over all of your accounts and confirming that they all agree with each other. (Also see debits and credits)