In E.J. Cand`es, D.L. Donoho, New tight frames of curvelets and optimal representations of objects with C2
singularities, Comm. Pure Appl. Math. 56 (2004) 219–266. on page 3 I find the following relation (eq. 1.1):
I tried detexify to identify the latex code for this symbol and work from there, but no success. What does this symbol mean?
Typically write $f\asymp g$ to mean $f$ is asymptotic to $g$, i.e., both $f\ll g$ and $g\ll f$. (This means the same as $f=O(g)$ and $g=O(f)$, or just $f=\Theta(g)$). This is how Graham, Knuth and Patashnik define $\asymp$ in Concrete Mathematics.
In other words, $f\asymp g$ means that there is a constant $C>0$ such that $$\tfrac1C\,g(x) \leqslant |f(x)| \leqslant C\,g(x)$$ for all $x$.
Have a look at this chapter explaining different asymptotic notations.