This means that the system makes (under certain circumstances) information available to an outside person. Such information could be sensitive user data (e.g. names, addresses, customer data, credit card details, etc.) or details about the system (such as the file system structure, installed software, configuration options, version numbers, etc). An attacker could use this information to craft an attack against the system.
There is a fine line between the protection against information disclosure and so called “security by obscurity”. Latter means, that system administrators or developers try to protect their infrastructure or software by hiding or obscuring it. An example would be to not reveal that TYPO3 is used as the content management system or a specific version of TYPO3 is used. Security experts say, that “security by obscurity” is not security, simply because it does not solve the root of a problem (e.g. a security vulnerability) but tries to obscure the facts only.