Re: What does this mean?
Bb13 is an extended dominant chord. In a normal Bb7 chord, you have Bb, D, F, Ab. Bb9 takes the 7th chord and adds a C, Bb11 adds an Eb, and Bb13 adds a G. The spelling of a Bb13, then, is: Bb, D, F, Ab, C, Eb, G. Now, if you were voicing a chord on piano, for instance, I'd caution you that you don't use all of those notes at the same time. But as a change in a solo, you can get away with using any of the notes. The fact that the 13th is specified says to me that the arranger is looking for the color of the extra tones (C, Eb and G), but if you're not comfortable with that, you can always get away with the standard Bb7.
Hope this helps,