Can someone explain center of mass in simple terms?

see saw

heavy person on one end
light person on the other end.

put the pivot at the center of mass so it balances.
Ignoring the mass of the plank itself

heavy mass (distance to c mass) = light person mass (distance to c mass)

in general
sum of m1 x1 + m2 x2 +....mi xi = (m1 + m2 +.... mi) x cm