A ledger is a running record of money in and money out, used by players to track deposits, withdrawals and net position.
A ledger is the only way to see whether you are up or down across an account history, because balances reset every time you deposit again.
500 euro deposited, 320 euro withdrawn, net position minus 180 euro, regardless of what the account balance shows today.
Because the account balance only shows the current moment. Deposits and withdrawals recorded over time show the net position, which is the figure that matters.
Licensed operators give players access to an account history or statement showing deposits, withdrawals and transactions. It is per operator, so a player with several accounts still needs one combined record. The personal play ledger tool on this site keeps that record in your own browser.