A leak is a small hole in a given technical system allowing the undesirable entry or exit of gases.
The leak rate describes the size of the leak in terms of the gas amount that passes through the leak per time at a given pressure difference.
No vacuum device or system can ever be absolutely vacuum-tight and it does not actually need to be. The simple essential is that the leak rate is low enough so the required operating pressure, gas balance and ultimate pressure in the vacuum container are not influenced.
The lower the required pressure level is, the more stringent the requirements in regard to the gas-tightness of an apparatus are.