Interpretation of the the solve() command

Hello,

Recently I switched from Mathematica to Maple. I have the following code 

solve({pf'(e)- psi'(e) - tau - mu*pi(tau)/(1 - pi(tau)) = 0}, {tau}) assuming 0 < p, 0 < tau, 0 < mu, 0 <= pi< 1

and I am getting a result of 

RootOf(-D(pf)(e)*pi(_Z) - mu*pi(_Z) + _Z*pi(_Z) + 2*pi(_Z) + D(pf)(e) - _Z - 2)

I have to questions: (a) what does that _Z means? (b) is it possible to express the above solution in a more "readable" way?

Thanks in advance