In simple terms, the fundamental principle of counting says that if I have m ways of doing one thing and n ways of doing another, the number of ways I can do both is

m*n.

Suppose I can play soccer or basketball or frisbee (3 games), and I can do any of them indoors or outdoors (2 places). There are 3*2 = 6 possibilities. They are

soccer indoors, soccer outdoors,

basketball indoors, basketball outdoors,

frisbee indoors, frisbee outdoors.

m*n.

Suppose I can play soccer or basketball or frisbee (3 games), and I can do any of them indoors or outdoors (2 places). There are 3*2 = 6 possibilities. They are

soccer indoors, soccer outdoors,

basketball indoors, basketball outdoors,

frisbee indoors, frisbee outdoors.