Different regions are based on the TV standards in various parts of the world. North America uses one standard, Europe another, Asia a third, and so on. DVDs produced for one region won't work on equipment made for a different region.
If you look on Amazon or Ebay for DVDs, then you will likely note that they might be produced for a specific region. North America is Region 1.
Here is a more detailed explanation,
Broadcast television systems - Wikipedia
BTW, we have several Goodwill stores in our area and I check them out several times a month to see if any DVDs of movies I'd like are for sale there. It seems that when the kids of older people clear out their parents houses the DVDs they don't want go to Goodwill. That's mostly what I and many others here would consider classic movies. Lots of John Wayne movies on DVD. Locally Goodwill charges $1.99 for DVDs or Blu Ray.
Last week I picked up "The Outlaw Josey Wales" and "Santa Fe Trail." This afternoon I picked up "MASH."