A Third Country National (TCN), is a person who is applying for a visa from within a country that is not their own.
For example, a Kenyan working and living in Mexico who is applying for a U.S. visa at the US embassy in Mexico would be referred to as a third country national.
A common version of what we see at RapidVisa is when an OFW (overseas foreign worker) from Asia is working legally in the Middle East and wishes to apply for a fiance visa from the embassy they are nearest to, rather than return to their home country and apply.
Some of the U.S. embassies and consulates require third country nationals to present their proof of legal residency in the third country when applying. Please check the local embassy or consulate for such restrictions on third country applicants.