Antenna gain is usually measured using the term dBi, which means 'the forward gain of an antenna compared to an idealized isotropic antenna'.
In plain english, high gain antennas work by concentrating radiation into a tight pattern, so you get a small area with a much stronger signal. Just think of a flashlight with a removable reflector and picture how removal or adjustment changes the pattern of light it produces (i.e. a bright, tight beam or dim, wide spread), and you will understand the concept involved.
The term dBi basically compares the strength of the point of the highest concentration of radiation compared to an antenna that radiates equally in all directions. Generally as antenna gain increases, the RF spread it provides decreases.
When expressed in dBi, it becomes very easy to calculate the output of the antenna at the point of its highest gain. Radios used in wireless networking usually have their output measured in dBm (decibels relative to one milliwatt).
To work out the equivalent output of an antenna for a given radio, simply add the figures; e.g. a radio outputting 12dBm (or 15.8mW) connected to an antenna of 15dBi gain gives a maximum output of 12 + 15 = 27dBm (or 501mW).
This does not take into account any signal loss through connectors and cabling, so in the real world this will need to be taken into account and subtracted from the equation in the same way.
This is about the easiest way I can explain it. I welcome anyone to correct any mistakes!
