Conquistadors were like soldiers and warriors and explorers who did the colonizing for Spain and Portugal through the 15th to 17th centuries.
Apart from this, it isn’t clear what the song is about, though politically, it has been interpreted in various ways. Given the 1967 date, around the Vietnam War, the song could be an anti-war statement: once shiny armour and sharp blades seemed the way to go in centuries past, for international relations and conquests. But in modern times, such accoutrements of war are dilapidated and rested upon by symbols of death, vultures.
Related: Neil Young, Cortez The Killer