Even though the answer seems obvious - the project is invite only - there is the question why you are seeing this.
When Project Owners setup invite only projects that are publicly viewable, it means anyone can see them.
Normally we do not publicise these projects in the discovery area because we want you to find cool projects to join. However a Project Owner may change the permission on a project from "Apply" to "Invite only"
Unfortunately if you see this there is nothing we can do. If you do not know the Project Owner personally or are not expecting an invite from them, then you are not able to join these projects.