One of the Agile principles states that "At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly." The Sprint Retrospective provides an opportunity for the delivery team to inspect and adapt by celebrating what’s working well and identifying specific actions the team may take to drive continuous improvement.
Similar to how the Sprint Demo provides feedback on the product produced during the Sprint, the retrospective (retro) meeting provides an opportunity for the team to give feedback on and improve its process. After reviewing data from the previous Sprint, the team identifies a small number (1-3) of concrete actions items that they can complete within the next Sprint. These action items become team commitments, just like stories, and the team will consider at the next retro whether they’ve had the intended result.
Retro Stages Source: Esther Derby & Diana Larsen, Agile Retrospectives - Making Good Teams Great