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.