Why is a polynomial in $z,\bar z$ analytic iff it does not involve monomials $z^i\bar z^j$ with $j\ge1$?

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I have encountered some note on complex analysis:
In particular, given a polynomial in the real variables x and y , with complex coefficients, these properties tell us that the polynomial is analytic on C precisely when, in its representation as a linear combination of terms $z^{j} \overline{z}^{j}$ , terms involving $ \overline{z}^{}$ do not occur.
what does it mean?Could you give me some formulas? enter image description here