Imagine an antenna that transmits signals outward in an expanding circle in all directions. No such antenna exists, but the theoretical or "isotropic" model forms the basis for a mathematical equation that calculates the "gain" or strength of a real antenna, which is measured in decibals (dB) and are referred to as dBi or dBd. The "i" in dBi stands for isotropic, and the "d" in dBd refers to a "dipole" antenna or an actual real-world antenna.
When calculating the signal strength, we begin with assuming that the isotropic or theoretical antenna has a gain of 0dB since its signal is transmitted equally in all directions. A real-world antenna will naturally have a higher dB or decibal that is measured in increments of 3dB. In reality, we don't need antenna signals going in every direction. Instead, we focus the antenna where it's needed most, generally in the direction of other network devices like bridges and access points.
Antenna gain is the ability of the antenna to radiate more or less in any direction compared to a theoretical antenna. If an antenna could be made as a perfect sphere, it would radiate equally in all directions. Such an antenna is theoretically called an isotropic antenna and does not in fact exist. However, its mathematical model is used as a standard of comparison for the gain of a real antenna.