The root of a polynomial f is a value for x where the polynomial value f(x) is zero.
For example, if the polynomial f ( x ) = 2 x 2 + 10 x − 28 {\displaystyle f(x)=2x^{2}+10x-28} then f ( 2 ) = 2 ⋅ 2 2 + 10 ⋅ 2 − 28 = 0 {\displaystyle f(2)=2\cdot 2^{2}+10\cdot 2-28=0} , so 2 is a root of this polynomial.
In fact, it can be seen that also -7 is a root of this polynomial.