The ideaTheory
A country's bank statement
Your bank statement lists everything that came in and everything that went out last month, and the closing balance tells you whether you ended up richer.
The balance of payments is that statement for a whole country against the rest of the world.
Money in from selling software: a credit. Money out to buy crude oil: a debit. If more went out than came in, something has to make up the difference, and that something is the reserve pile in the vault at the Reserve Bank.