The Beacon it's the Utopians Ultimate Weapon. Like the name suggests, it's something that is going to patrol a certain region of the universe. And it's exactly that.
When you place a Beacon it will, depending on its level, give information of the surrounding Planets and Fleets. It also prevents Devastations to the fired in the area covered by the Beacon.
Its level is given by the sum of the levels of your Deep Space Scanner. Example: if you have 3 Deep Space Scanner level 1 and 1 Deep Space Scanner level 2, then your Beacon level will be 5.
A Beacon can be created by any fleet. If all conditions are met (necessary resources and, depending of the level, if you're near a planet that you own), the folowing option will appear in the fleet popup:
The picture below illustrates the information of all the levels: