Where does this symbol come from? Or is it just a show? Or how is it related to the game?

Where does this symbol come from?

What you are seeing is known as a Crop Circle. A very brief explanation is that Crop Circles are commonly associated with the existence of aliens, and they have recently visited the area.

How is it related to the game?

According to this article, these crop circles belong to a series of side quests to obtain the alien weapon Magnopulser.

1. Free Larry

   Larry has been trapped in an energy field that is controlled by three nearby generators. Locate each one and turn them off.

2. The Hero's Journey

   Larry is pretty paranoid about cultists listening in on conversation and wants the listening devices destroyed on the PurpleTop Telecom Tower. The travel point is disable for the mission so players will have to get there on their own. The tower is guarded by cultists and a dog. Once the area is cleared, look for the grapple point on the first upper platform. Use the ladder to get to the second, and then the third. Shoot the first listening device. Grapple to the next platform and shoot the second device. At this point, Larry will radio the player and bring in a helicopter. Climb to the next platform. Grapple to the skid and climb up into the chopper. From the inside of the chopper, shoot the last three.
   
   Larry will land the helicopter near his place, but cultists have decided to pay it a visit. Players will need to secure the area. Talk to Larry again to end the mission.

3. Close Encounters - This is the quest with the crop circles

   Larry need four glowing alien objects brought back to him. The first two are in a nearby field with crop circles in it. There are enemies in the field so watch for those. The first waypoint leads to a glowing cow. Remove the green object, then head for the second waypoint. Watch for enemies with rocket launchers. The next glowing object is in the center of the circle surrounded by turkeys. They will attack players once the object is removed.
   
   The third object is close to some hay barrels, but a dog will grab it and run off. Follow it till it drops it, or just kill it. The last object is on top of a grain silo with a green roof. Eagles will be flying overhead and the object is in their nest. To get to it players will need to use the nearby water tanks. Climb up the ladder, then jump to the roof of the second tank. From there, jump to the silo, landing on a platform. Climb that ladder to the next level up, then look for ways to keep going up. Return the items to Larry.

4. Out Of This World

   Talk to Larry
   
   Talk to Larry to begin the final phase. Now that he has the artifacts and >!the listening devices are disabled, he's free to teleport to Anne in space.
   
   Unfortunately, there's not enough power to send him out, so he needs you to >!re-route the grid to his lab. Go to Hilgard Electric Power Station
   
   The nearby Hilgard Electric Power Station to the Northwest can route the >!power, so grab a vehicle, or fast travel.
   
   Upon your arrival, flip the first breaker in a small shed. Unfortunately, >!this sounds an alarm, bringing attention from Peggies who will show up to >!check it out.
   
   Larry will tell you to hurry up because there's no time, but this is not >!the case. Dispatch the Peggies, and then flip the next breakers.
   
   The breakers must be thrown in sequence, so follow the markers. The fifth >!breaker is on an overhead gantry, with a zipline going to the sixth and >!final breaker.
   
   After the sixth breaker is flipped, head back to Larry. Return to Larry
   
   Larry congratulates you on your ability to flip switches, and will ask you >!for one last favour: power up the teleporter to send him on his way, as he >!can not do it himself from inside the field generator. Power O
   
   Hit the button on the generator to activate the Dynamic Laser Capacitor.
   
   After this firing sequence, the Magnopulser and Larry Parker's Pants will be in the containment field.