The term "leak" can be used for several different purposes.
An information leak means that unexpected people gain access to secret or confidential data and information. This can happen intentionally through hacking or through inadvertent actions, such as an employee accidentally sending confidential email to the wrong person. Information leaks can damage the company's reputation as well as lead to cyber-attacks so it is important to prevent them by taking appropriate security measures.
It could also be a leak of user data. In other words, your data has somehow become accessible to an unauthorized person. Even this can see through hacking or accidental actions. If this happens, someone may gain unauthorized access to your data. If this happens, it is incredibly important to take action as quickly as possible.