∏ — product

$\prod_{t=1}^{T} a_t = a_1 \times a_2 \times \cdots \times a_T$.

It appears in two places in the course. First, chaining probabilities: a sentence's probability is the product of each token's probability given the tokens before it. Second, chaining derivatives through many steps. This is why RNN gradients vanish: multiply $T$ numbers that are each below 1 and the result shrinks toward zero, faster the larger $T$ is.