When I studied the expectation of a discrete random variable $X$, I saw it is defined as $$\mathbb{E}(X) = \sum_{x \in \text{Img}(X)} x \cdot \mathbb{P}(X=x)$$
What exactly does $x \in \text{Img}(X)$ mean? Does it mean $x$ can be all possible values of $X$?
The set of all possible values the random variable $X$ can take on. For instance if $X$ represents rolling a dice, Img(X) would be $\{1,2,3,4,5,6\}$