Think of it as a light bulb. When the bulb is lit, light goes everywhere. If you place a reflector behind it the light is concentrated but at the expense of shining uniformly around. Antennas are very similar. An antenna with unity gain will radiate (or receive) equally in all directions (think of the pattern as a doughnut laying on a table) . If you narrow the radiation pattern you can create an effective gain as in a beam antenna. You can also in effect do this by flatting the doughnut and achieving an omnidirectional pattern with gain. Antenna gain is expressed in decibels (dB).