In Federation society, a legal guardian was a person responsible for a person unable to operate as a responsible adult themselves, the latter a ward of the former. They were tasked with making decisions and looking after those persons, with their best interest in mind.
In 2269, Flint claimed Rayna Kapec was placed into his custody as a legal ward after her parents died. However, no record of this could be found in the Federation legal banks. (TOS: "Requiem for Methuselah")