The B flat eleventh chord is a 5-note chord consisting of the notes Bb, F, Ab, C and Eb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bb11.
Chords that are a superset of Bb11. The chords include more notes but always Bb, F, Ab, C and Eb.
Chords that are a subset of Bb11. These contain less notes but all of them are included in Bb11.
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 Bb11 chord. Feel free to save or share the image as needed.