The F diminished seventh chord is a 4-note chord consisting of the notes F, Ab, Cb and Ebb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Fdim7.
Chords that are a subset of Fdim7. These contain less notes but all of them are included in Fdim7.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Fdim7 chord. Feel free to save or share the image as needed.