Fleet Admiral is the highest rank a player can receive in Star Trek Online. It is granted when a player reaches level 60. It was one of 2 new ranks added to the game in Season 10: The Iconian War (the other being Admiral).
Prior to Season 10, a player could only reach Vice Admiral status when they reached level 50.
As a reward for achieving level 60 and Fleet Admiral status, the player gains a new trait slot. It also unlocks the Fleet Admiral pip/bar for a character's uniform, which can be added to a players uniform in the tailor.