A zero polynomial is a a constant polynomial, whose coefficients all equal 0. There are an infinite number of 0's in a zero polynomial. Any value of x can be a zero of a zero polynomial, which is why there are an infinite number of 0's in a zero polynomial.
Think about it. The 80 is the y-intercept meaning that is the starting point or starting height where the rocket is launched from. The purpose of the expression is to show the height of the rocket over time.