Intuitively, gain can be defined as a ratio that represents the amount by which an antenna boosts the power output of a signal as compared to an isotropic configuration (or at least usually - sometimes gain can be given in units like dBd, which represents the gain relative to a dipole antenna). Formally, gain is defined as $G=\epsilon_RD$, where $\epsilon_R$ is the radiation efficiency of the antenna, and $D$ is the directivity. Given that it's so closely linked to directivity, we can see how high gain isn't always a great thing. High gain is useful for when signal direction is easy to predict. In comparison, low gain is useful for when a more omnidirectional configuration is optimal.