In number 1, the center cell only has one

living neighbor, so it would turn white.

In number 2, the center cell has three living neighbors, and here we see it remains alive if

two or three neighbors are alive, so

it would be black at the next time step.

And on this one,

there's four neighbors,

so we get overcrowding

for the middle neighbor,

and not two or three, but

four neighbors are alive,

so this would be white at

the next time step.