Adjoining a root of an irreducible polynomial
= Adjoining a root of an irreducible polynomial
If $f\in k[X]$ is irreducible, then $(f)$ is a maximal ideal and
$$
k[X]/(f)
$$
is a field extension of $k$. The class $\alpha=X+(f)$ is a root of $f$, so this quotient adjoins a root of $f$ to the base field.