The data presented here measures:
Proportion of people who perceive their rights to land as secure, out of total adult population (%).
This is a portion of the SDG indicator 1.4.2 which captures the perception aspect.
The data is managed by UN-Habitat and the World Bank, and the original dataset can be downloaded here.
For more details on how SDG indicator 1.4.2 is measured, consult this page.