Stability
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Stability is one of the Game Mechanics in Final Factory. Most structures in the game have a stability cost which is equal to the number of tiles they occupy. So a 
Solar Panel takes 2 stability and a 
Station Core takes 9.
Each station gets 50 free stability to work with before the player will need to include stability increasing structures to the station. A single Station Core can confer a large amount of stability to a station. Additional Station Cores can be connected to a station, but only the first one will add full to the stability after that every newly placed Station Core only gives a portion of its normal stability bonus.
Stabilizer Artifact can also add a flat amount of stability to a station, but they also gain the reduced stability bonus if more than one is used.
Further, 
Struts can be added to a station to decrease stability usage of structures it is connected to. When connected between 2 structures, it will reduce the stability cost of the smaller station by up to 5 stability (if the station takes less than 5, it will only remove up to that station's stability cost). When connected to large structures, multiple Strut can be added to reduce the stability of a station beyond 5.
But what is stability?
One can think of it as an abstract combination of structural integrity and ease of station keeping.
Every structure has very weak EM station keeping thrusters (also used for the 8m/s movement speed) to counteract the influence of decaying orbits, micro-asteroid impacts, enemy fire, and of course movement that threatens to move it somewhere else or send it into a dangerous spin that could tear the station apart.
Struts reinforce stability despite not being in any way involved in movement. Why?
Because when a structure connected only by frail and hollow conveyor chutes gets a nudge or is moved in only one end, it has a very high risk of just tearing itself apart. When one side of the station gets going with high momentum and tries to drag the rest of the station with it the frail chutes will simply tear open rather than bring its side of the station up to the same velocity.
But if it's connected by thick, massive steel columns a lot more force is needed to tear the structure apart. The stability artifacts look like fancy gyroscopes, which is helpful to prevent the station from spinning out of control, but they may also spread something akin to the Integrity Fields of Star Trek, literally increasing the strength of the electromagnetic bonds that bind molecules together.
Thanks to @the Meanie for this explanation