It's a measure of how much the aerial concentrates the radio waves compared to a standard dipole aerial (ie a simple stick), which more or less radiates and receives in all directions.As I understand dBi it is a measure of electrical gain – i.e. the amount by which the aerial/antenna magnifies the incoming signal. In this case, more dBi should always be better. Is it as simple as that?
Omnidirectional ones concentrate them into the horizontal plane, making use of the waves that would otherwise go up into the sky or down into the ground.
Directional ones concentrate them in one direction in the horizontal and vertical planes.
10 dBi is 10 times better, 20 dBi is 100 times better.