How do historians translate and make sense of old documents when they're in such bad shape?

I'll use the Dead Sea Scrolls as an example. There are so many holes in them, one even has the whole bottom part of the scroll missing.

It also reminds me of the Rosetta Stone. A lot of it is missing.

edit. I remember from foreign language classes how getting one letter wrong can throw off the entire meaning of a sentence or paragraph.

Or this. There could be something important in the top part of the page that is missing.