I have started seeing the "$\epsilon$" symbol in this paper. What exactly does $\epsilon$ it mean in the formula?
The hard part is this definition: "Range$(p,\epsilon)=\{o\mid ||p,o||_2\leq \epsilon,o \in \mathbb{R}^d\}$." I don't know what $\epsilon$ means here. FORMULA IMAGE
The symbol is called "epsilon," and is simply a variable, just like $x$ or $y$. However, note that $\epsilon$ is most commonly used in analysis as an arbitrarily small (but positive) value.
If you are new to the idea of an $\epsilon$-neighborhood, check out this post.
Be sure not to confuse "$\epsilon$" with "$\in$", which means "element of." These symbols both looks like "e"s, but have much different meanings!