Fb is built from Fb, Ab and Cb. Fb is the root, Ab the major third and Cb the fifth. Said aloud, its notes are F flat, A flat and C flat.
String by string, low to high, it plays Fb, Cb, Fb, Ab, Cb and Fb. Of the six strings you play, three give Fb, the note that names the chord. The lowest and highest notes are both Fb, two octaves apart, on strings 6 and 1.
The open strings add notes to Fb too. String 6 gives Fb, string 2 gives Cb and string 1 gives Fb. Your hand covers two frets, from 1 to 2. There is one more position, starting at fret 7.
Change only string 3 in Fb and you have Fbm. Fb and Dbm have Fb and Ab in common. Moving to E7, you keep two fingers down. E is the same chord under another name.
You will find Fb in seven different keys. In E major it is the I, in A major the V and in B major the IV. The rest appear in the key list.