dBi is dB(isotropic).
It is the forward gain of an antenna compared with the hypothetical isotropic antenna. An isotropic antenna radiates uniformly in all directions over a sphere centred at the antenna.
When the antenna is connected to a radio source, the gain is expressed in deciBel (dB). x = 10 log10 (Po / Pi) where Po is the output power and Pi is the input power from the radio source.