With any project there are lessons which are learned. These lessons could be about something good that changed the course of the project or the lessons could be mistakes which should have been avoided. A lesson could be about the experience of project team members and how those members made decision which helped avoid big mistakes. Other lessons learned could be about the schedule and how ordering certain material from a vendor helped reach certain deadlines. Lessons could be simple as a type of ad hoc meeting which was held weekly to ensure strong communication.
Whatever the lessons might be, it is important to record them, so those lessons can be taught to others on future projects. Without a record of the lessons learned then future project managers and project team members will not be able to learn.
Most importantly, without a record for future projects, then mistakes will surely be repeated. The lessons learned register captures the situation, the impact, challenges, and possible recommendations. The register can be populated as the project is happening and does not need to wait till the project closes. The register should periodically be brought up in project meetings to remind people to capture lessons learned.