I often enjoy reading on a topic and then try to summarize it in a short article on this website. But today isn’t the case. Sometimes a simple instruction video does a much better job than all the words I could come up with. Aruba Networks, manufacturer of wireless devices, made an excellent short learning video clip that explains what “Antenna Gain” is and help you visualize how different antennas create different RF patterns.
Although their products are targeted toward broadband internet wireless control and access, the theory demonstrated by the Aruba engineer in this video also applies to Amateur Radio antennas, as radio waves are transmitted and received the same way on every frequencies.
The video above gave a very good understanding on how an antenna directs it’s energy. It’s all in the design of the antenna one might say. You can always increase the energy output of the transmitter to achievement stronger electro magnetic fields, but if not directed, it’s will be mostly wasted energy. So, how do you increase this directed energy without increasing the transmitter output ?
It cannot be said enough that your antenna is the single most important piece of equipment that will have the most profound impact on your whole radio experience, therefor, your antenna is where you should spend the most time and effort to understand and build properly. Also, it’s a lot of fun. 
In a YAGI beam, the radiating element is what you saw in the video above. The ballon. But with careful calculation and positioning of other elements near the radiating element, your RF signal will be reshaped and sensitivity (Both in transmission and reception) will be increased proportionally to the number of elements used. Well, almost.
Each elements will absorb the RF pulse of energy (Unit of Frequency) and then resonate some of that energy back, acting as another radiating element. Some of the energy will be lost during this process, but some of this energy re-radiated will be carried forward to the next element, adding itself to the original RF energy wave of the original radiating element, increasing its amplitude, therefor giving more “Gain” in a specific direction. The opposite is also right when receiving a RF wave. Each element will absorb the incoming RF wave, loose a bit of that energy to resonate back to the receiving element the signal. The more elements you have, the more the amplitude will increase, giving achieving increased reception “Gain”.
73′s.