How to interpret this formal language definition?
This is pretty standard notation for

$\qquad \#_a(w) = |w|_a = $ number of occurrences of $a$ in $w$.

Building a PDA for this language is straight-forward. Find a hint below.


   Use the stack to count $\#_0 - \#_1$ in the processed prefix of the input. Figure out how to model negative numbers.