What is the meaning of the above symbol (the circle with the cross)? I frequently come across it, but as I am learning mathematics on my own, I wasn't able to figure out where it comes from and what it means ($\mathcal{A}_i$ are $\sigma$-algebras and $I$ is an index set). It is obviously some kind of "product" but as it is acting on families sets, it can be neither the normal dot product nor the cartesian product (which looks different anyway). I have also seen it in contexts of product measures where it acts on $\mathbb{P}_{X_i}$, which are probability measures on random variables.
What does it mean and what does it do? Where can I learn more about it?