Y=a (x-r) (x-s) what is a and r and s?

The constant a is the scale factor on the parabola; positive a makes a CUP (concave up), which is the usual orientation. Negative a makes an upside-down parabola. Big a (in absolute value) makes a skinny parabola, small a a wider one.

r and s are the zeros of the quadratic function, the values of x that give y=0.

a, r, s, x all are variables