A certain radioactive substance decays such that the amount of substance remaining, in grams, can be modeled by the function $M(h)=500(0.9{)}^h$, where $h$ is the number of hours since the initial measurement was taken, and $0\le h\le 24$. If $y=M(h)$ is graphed in the $hy$-plane, which of the following is the best interpretation of the $y$-intercept of the graph in this context?