Right to build is an ownership title for real estate and land in Indonesia. You have the right to build property on the land, but also to keep it. It doesn't have to be built on indefinitely to keep that title.
Right to build can be obtained by buying “Freehold” land or buying “Right to build” land from someone else. The big advantage of “Right to build” is that even as a company that is 100% owned by foreigners, you can still own the land. In the past this was not possible. When you apply for “Right to build” it is valid for 30 years, after which you can immediately extend it for another 20 years.
Another form of ownership that is often used by foreigners is "Leasehold".