It's the song that plays during the bridge-crossing scene in the first game. It's hard to pin a definite name on it since it usually sports a different title from game to game, from "Opening Theme" to "Prologue" to simply "Final Fantasy." Furthermore, FFVI started a tradition of mixing it into the song that plays during each game's ending credits instead of letting it stand on its own. (FFXII thankfully diverts from this pattern, although you can bet it'll be reinstated for FFXIII, if the theme is even allowed to make an appearance at all.)