The + indicates that the 5th is augmented.
The regular Bb7 consists of the 1st, 3rd, 5th and flattened 7th notes of the Bb scale.
The augmented one is the same except the 5th is sharpened, from F nat to F#.
Bb7: Bb (I) D(III) F(V) Ab(bVII) - fingering example 1,2,1,1 (bVII, III, V, I)
Bb7+ Bb(I) D(III) F#(V+) Ab(bVII) - fingering example 1,2,2,1 (bVII, III, V+, I)
Hope this helps.