Could someone please explain to me what cross chaining is? I assume it's when you are in the big ring in front and the small cog in back but I'm not sure. Is it really bad for the chain? If so, why? Or is this something the chain manufacturers made up to increase sales?
Nono, I don't think this is what the manufacturers are doing to increase sales. Nono!
They have no point in doing that. In fact, manufacturers like Shimano actually advises us not to us the biggest ring in the front and the biggest gear at the back, similarly for the smallest in the front and at the back. This is as the chain will be slanting a lot and cause wear and tear more easily. Note this shape :
Biggest in front
and the back
Smallest in front
and the back
Understand? It really isn't made up, simple logic will tell that it will wear faster, after you get the visualisation right.
Cross chaining is using one of the following gear combos: Big ring and big cog or Small ring and small cog.
It puts a lot of stress on the chain because of the extreme angles. This is a result of drivetrain manufacturers stuffing far more gears than are necessary in the rear cluster. Ironically, they give you extra gear combos but you can't utilize them all.