Peer learning days are mandatory days in which students come together to review the project(s) they have been working on. They start promptly at 9 am, and groups of 6-10 students are pre-assigned. Students spend the entire day going over the project(s), talking about concepts and asking questions. If there are group members who weren't able to complete all of the tasks, this is the time for their peers to help them out - not by giving answers, but helping them find the answers themselves. This can be done by students who completed the task successfully talking about their thought process and asking others to share theirs, for example.
Holberton is much more focused on comprehension than completion - it is very typical that the majority of the group got the entire project done, but that does not mean that you understood all of it. Peer learning days are about talking though any confusion and finding clarification.