Antenna Gain is a measure of basically it's how much more "powerful" the antenna is over an imaginary dipole antenna. I say imaginary because nobody makes one, but it would be an antenna that spreads the signal in every possible direction, in the shape of a sphere. Such an antenna would have zero Gain. So for example, a 24dBi gain parabolic grid is a very focused antenna, it narrows the signal into a fine beam of coverage in one direction. It isn't actually more "powerful" than a dipole, it just focuses all that signal into one spot, so it goes farther.
A good analogy would be that of a torch. Unscrew the cap and lens so you just have a bare bulb, and your immediate area will be quite bright, but it doesn't go very far. This would be akin to a 1 or 2dbi antenna. Now put the cap back on and focus the lens into a tiny little spot, and shine it down a hallway. Your immediate area isn't very bright, except for that one bright spot down the hall. This would be akin to a 24dbi parabolic dish.