I'm new in SonarQube I started reading documentation but a lot of time a found "The leak period" but I didn't found anything about it can someone explain me what it means.
Shortly, leak period is time frame (usually since last release), where specified criteria are measured on newly added code. This allows to focus on quality of fresh code and stop the accumulation of technical debt.
The "leak" concept is explained in documentation here https://docs.sonarqube.org/display/SONAR/Fixing+the+Water+Leak
Update SonarSource has fleshed-out and updated the terminology / philosophy: https://sonarqube.org/features/clean-as-you-code.