Sorry please discard my previous message. I read that as Bug interval period.
Build interval period is the time between two builds released for testing..
(Like Build 200 Build 201...)
During this period testers will be testing on the build which has been released & Developers will be working on 1. the build which is going to be released. & 2. also on bugs which are filed by the testers for the build which was released.
So both the team will be busy. This saves in Resources & time. & improves Quality