What is the meaning of Skewness?

The property of being skew.

A measure of the asymmetry of the probability distribution of a real-valued random variable; is the third standardized moment, defined as $\scriptstyle \gamma _{1}={\frac {\mu _{3}}{\sigma ^{3}}},\!$ where $\mu _{3}$ is the third moment about the mean and $\sigma$ is the standard deviation.