what does _Z and Z_^2 mean?

I'm solving this equation:

eq3 := 2*lambda*(q[1]+p[1]*exp(t))*(q[2]+p[2])^2*p[1]*exp(t) = s[1]

eq4 := 2*lambda*(q[1]+p[1])^2*(q[2]+p[2]*exp(u))*p[2]*exp(u) = s[2]

solve({eq3, eq4}, {t, u})

This is what I get:

{t = ln(RootOf(-s[1]+(2*lambda*p[2]*q[1]+2*lambda*q[1]*q[2])*_Z+2*lambda*_Z^2)/((q[2]+p[2])*p[1])), u = ln(RootOf(-s[2]+(2*lambda*p[1]*q[2]+2*lambda*q[1]*q[2])*_Z+2*lambda*_Z^2)/((q[1]+p[1])*p[2]))}

I'm completely new at maple.

I can't find what _Z  and _Z^2 mean. Can anybody help? and how I can solve this _z and _Z^2

Thanks in advance