A F♯ Major 6 (F♯6) chord contains the notes F#, C#, A#, D#. It is built from the major 6th formula 1 - 3 - 5 - 6, with F# as the root.
Why do bossa nova players use 6th chords so much?
Because a plain major triad sounds too final for music that keeps moving. Adding the 6th softens the arrival without introducing the tension of a 7th, so a phrase can settle without stopping. In bossa the 6th is often the default major chord, with the plain triad reserved for the very end.