As shown in the image below, how does A turn into B? What is the intuitive logic behind the disappearance of sigma y? Thanks!!

As shown in the image below, how does A turn into B? What is the intuitive logic behind the disappearance of sigma y? Thanks!!

The event $\{X=x\}$ is the union of the mutually exclusive events $\{X=x,\ Y=y\}$ for all $y$.