what does burn-through mean?

I read it as the ink leeched through the page, so it was visible and made it difficult to read the text on the opposite side.

Decay/ resulted in > discoloration, migration, diffusion, burn- through, loss

“Burn through”
is a term that describes what happens when heat creates a certain look or quality on paper. Crafters try to create this look to make the paper look old.

Oxygen
Sunlight
That yellow light inside your home. ( Sometimes florescent light )
Exposing to the elements
Cigarette smoke.
Chemicals from various metals. Like keeping papers in specific safes could result in those papers yellowing even if protected.
That being said not storing in a good place.
Removing from storage.

The cheapest storage can go a long way