Is there a simple argument that shows that  the unique games conjecture implies the PCP theorem how can one show that what is relation between "Unique games conjecture" and "PCP theorem"?
how does one explain "Unique games conjecture" is stronger form of "PCP theorem"?
It depends a bit how you define the PCP Theorem, is it with perfect completeness or not. As we state in our book, an equivalent form of the PCP Theorem is that there is some constraint satisfaction problem for which it's NP-hard to distinguish between the perfect satisfiable case and the case that one can satisfy at most some fraction $s&lt;1$ of the cosntraints. But, we could have state a variant with imperfect completeness, replacing the perfect satisfiable case with being able to satisfy some fraction $c&gt;s$.

The unique games conjecture is an assumption of this latter form (where, making the condition stronger $c$ is close to $1$ and $s$ is close to $0$) and, most importantly, the constraints are of a very special form (permutation constraints on two variables). In that sense it's (essentially) stronger form of the PCP Theorem. 

You can ask if there's an easy transformation to convert a PCP with imperfect completeness to one with perfect completeness. I think it probably can be done easier than proving the PCP Theorem, but I'm not aware at the moment of a very simple argument.