Substrate Concentration
Substrate concentration is how much of a starting material is available for an enzyme to work on.
Think of it this way
A mixing bowl sits on a counter. Inside, a few dough balls wait to be kneaded. One pair of hands can only work on so much at once. Add more dough balls, and the hands stay busier — up to a point where they are always working.
Explaining substrate concentration by grade level
Think about washing greasy dishes. A little soap breaks down a little grease. Add more grease to the same water and the soap works harder. If you add too much grease, the soap can not keep up.