I do not have a strong math background and I have come across the symbol in some equations in Machine Learning.
For example, the following equation in Mnih et al 2013: $$Q^∗(s, a) = \max_\Pi \Bbb E [R_t|s_t = s, a_t = a, π]$$
Or this equation in the CycleGAN paper: $$ℒ_{GAN}(G, D_Y, X, Y) = \Bbb E_{y~p_{data}(y)}[\log D_Y(y)] + _{x~p_{data}(x)}[\log(1-D_Y(G(x))]$$
Can someone please help me understand the meaning of $\Bbb E$? (I'm sorry for bad English, it isn't my first language)
This is the expected value, known informally as the mean or (ambiguously) as "the average". For a discrete random variable, it's the sum of the possible values a variable can take multiplied by the probability of that value occurring, e.g. $\mathbb{E}(X) = \sum_{n=0}^\infty x\text{P}(X=x)$.
The pipe character $|$ indicates conditional expectation e.g. $\mathbb{E}(X|Y=10)$ means "the average value of $X$ in situations where $Y=10$".