It is not Spanish but Portuguese, means "plateau", and is part of the name of several geographical places. But I guess if you are here you are referring to the Palácio do Planalto ("Planalto Palace", which is how the seat of the Brazilian Federal Government is known, in its capital Brasilia. Although officially the name is Palacio de los Despachos .
Name of the Presidential Palace of Brazil, that is, its seat of government. In Brazil, it is the name of a municipality belonging to the State of Rio Grande do Sul. It is also the name of an ecoregion of Brazil, with very special characteristics : it is an extensive plateau and contains the large massif. It is also called the Brazilian Plateau.