Solution vs Primitive

Question

What is considered as a solution of the Pfaffian DE $Pdx+Qdy+Rdz=0$? If the LHS is the total differential of a function $F(x,y,z)$ then $F$ is called the primitive of the Pf DE. My book says $y=y(x)$, $z=z(x)$ is a solution of the Pf DE is they reduce the DE into an identity in $x$. So what does it really mean for something to be a solution of a DE? I thought that $F$ could be called a solution.