"Sign off" is not a Agile term per se, but during the SPRINT REVIEW you are demoing the latest build/functionality/features to the stakeholder/product owner. Based on this, the customer may say keep going, or I want to change direction, or I just thought of a new idea! All of these inform the stories in the backlog, which we will then reprioritize, add to, update, etc to get the next block of stories to be in the next sprint.