The B flat altered chord is a 4-note chord consisting of the notes Bb, D, Ab and Cb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bbalt7.
Chords that are a superset of Bbalt7. The chords include more notes but always Bb, D, Ab and Cb.
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 Bbalt7 chord. Feel free to save or share the image as needed.