I am studying PDEs whose symbols satisfy  

\begin{equation}
 |\partial^\alpha\_\xi\partial^\beta\_xp(x,\xi)| \lesssim M(x,\xi)\Psi(x,\xi)^{-|\alpha|}\Phi(x,\xi)^{-|\beta|}
\end{equation}
for all multi-indices $\alpha,\beta$ and $(x,\xi) \in \mathbb{R}^{2n}$.  

It is said that we must look at the metric
\begin{equation}
g\_{x,\xi}=\frac{|dx|^2}{\Phi(x,\xi)^2}+\frac{|d\xi|^2}{\Psi(x,\xi)^2}
\end{equation}
for the phase space to study such PDEs.  

My question- How do I relate the symbol to the metric? What is reason behind such a choice of metric?



Metric on the phase space
You can in fact reformulate the conditions on the symbol $p$ by saying that
$$
\vert p^{(k)}(X) T^k\vert\le C\_k M(X)g\_X(T)^{k/2},
$$
with $X=(x,\xi)$. Here $p^{(k)}(X) $ stands for the $k$-th derivative (a $k$-multilinear form) and $T$ is a vector in $\mathbb R^{2n}$. Using Lars Hörmander's notations in Chapter 18 of the third volume of ALPDO (Springer-Grundlehren 274), you have
$$
p\in \mathcal S(M, g).
$$
The asset of this presentation is that you can for instance give rather simple sufficient conditions on the metric $g$ to ensure that symbols in $\mathcal S(1, g)$ give rise to bounded operators on $L^2(\mathbb R^n)$. Also, you can prove that you have an algebra of operators and results of type 
$$
P\_1P\_2\quad \text{has a symbol in $\mathcal S(M\_1M\_2, g)$},
$$
whenever $P\_j$ has a symbol in $\mathcal S(M\_j, g)$, under some rather simple conditions on the weights $M\_j$. You may have as well some asymptotic calculus and define a "Planck" function
$
h(X)
$
which in your case is
$(\Phi(X)\Psi(X))^{-1}
$
(which is required to be $\le 1$, some type of Heisenberg Uncertainty principle)
and show that, for $P\_j$ as above with with symbol $p\_j$, 
$$
P\_1P\_2-\text{Op}(p\_1p\_2)\quad \text{has a symbol in $\mathcal S(M\_1M\_2 h, g)$}.
$$
The first occurrence of this type of conditions was given by Richard Beals and Charles Fefferman in their solution of the Nirenberg-Treves conjecture for local solvability of principal type differential operators: they had to resort to a non-homogeneous micro-localization scheme involving the metric in your question in order to obtain some factorization result which fails to hold with standard homogeneous microlocalization arguments.


