Hello, Bloody_dark -
Often those who rent DVDs or borrow them from the local public library mishandle them and do not know how to handle them properly.
From your question, it seems the DVDs are either scratched or dirty (possibly with food residue.)
This website gives step-by-step instructions on how to clean the readable side of a DVD:https://www.wikihow.com/Clean-a-DVD
If, after cleaning, the DVD still is grainy, patchy, skipping, etc., it is probably scratched.
What I do when I have a rented or public library DVD which is scratched, I return it with a "sticky note" attached to the DVD box stating it is scratched and that I properly cleaned residue from the readable side of the disc, so that the rental business or the library staff will know that the DVD is damaged and can choose to replace it.
There are ways to remove scratches from DVDs, but I do not recommend anyone try those steps if they do not own the DVDs themselves.
Note: I am a librarian by profession and have seen firsthand how people mishandle DVDs. I'm from the 45-rpm and 33-rpm vinyl record generation. We learned how to properly handle those vinyl records so as not to scratch them. Bottom line: We were taught to only handle the vinyl records (and subsequent CDs and DVDs) by their outer edges. For CDs and DVDs, they can be held between the thumb and forefinger, along the inside and outside edges.