Antennas come in all shapes and sizes with their own gain values and intended purposes.
Antenna gain is a comparison of an antenna against a isotropic (perfect) antenna is measured in dBi.
The isotropic antenna does not actually exist, as a perfect antenna can not be physically made. To visualise an isotropic antenna it can be imagined as a tiny round point with waves being radiated equally in all directions in the shape of an expanding sphere.
Most physically antennas are not like this and do not radiate in a perfect sphere like an isotropic antenna. To visualise a normal antennas radiation pattern two slices are taken out the radiation pattern to gain a visualisation of how waves are radiated.
The first slice is known as the H-Plane, showing a top down view of the radiation pattern from the centre of the antenna.
The second slice is known as the E plane, showing a side view of the same radiation pattern.
These slices can plotted onto polar plots, giving a visual representation of the signal strength of the antenna in all directions. The further away the plot point from the centre (where the antenna is normally plotted) the better the signal.