I just grabbed it from Wikipedia, so it’s likely written to be understandable to a layman (which is good, because that’s what I am).
That said, it’s just referring to the mass relative to the point you’re declaring the center of mass. If there’s 15g on the left, there should be 15g on the right; think of it like the center of mass being (0,0,0) on a 3 dimensional graph. 15g on one side is “negative” and on the other is “positive”, so the sum at the (0,0,0) point is 0g.
I just grabbed it from Wikipedia, so it’s likely written to be understandable to a layman (which is good, because that’s what I am).
That said, it’s just referring to the mass relative to the point you’re declaring the center of mass. If there’s 15g on the left, there should be 15g on the right; think of it like the center of mass being (0,0,0) on a 3 dimensional graph. 15g on one side is “negative” and on the other is “positive”, so the sum at the (0,0,0) point is 0g.