I think the person meant that, to a single x, they map more than one value. A function needs to have at most one image to the antecedent. In other words, it can

I think the person meant that, to a single x, they map more than one value. A function needs to have at most one image to the antecedent. In other words, it can't have "vertical stretches". This is different from bijectivity (specifically, injectivity here) which asks that two different x values never map to the same y.