Passports function as a key to the outside world, but passports from certain countries significantly limit how many places their holders can access. Citizens of many developing countries and of nations ravaged by conflict have a significantly harder time crossing borders than those of most other nations.
Nomad Capitalist, an organization focused on worldwide asset diversification, created the Nomad Passport Index as a way to rank countries based on the usefulness of their passports. Specifically, the index ranks countries based on how easy it is for their citizens to move freely between countries. It also considers their exposure to overseas taxes as well as the rights they are granted domestically.
A country allows entry to people based on their passport, trusting that the issuing country does a good job of ensuring its citizens will conduct in a safe and responsible manner. But citizens of countries with weak passports often require a extra levels of vetting.