The notes of F#m7b5 are F#, A, C and E. F# is the root, A the minor third, C the flat fifth and E the minor seventh. Said aloud, its notes are F sharp, A, C and E.
String by string, low to high, it plays F#, E, A and C. It runs from F# on string 6 up to C on string 2.
F#m7b5 uses no barre and no open strings, so you can slide the shape up the neck without changing fingers. Strings 5 and 1 stay silent. Your hand covers two frets, from 1 to 2. There is one more position, starting at fret 9.
Change only string 4 in F#m7b5 and you have F#dim7. Inside F#m7b5 sits a complete Am chord, made of A, C and E. Moving to E, you keep one finger down.
F#m7b5 shows up in two keys. In G major it is the viiø7 and in E minor the iiø7. Further down there is a song that uses it.