What does MOD mean?
Manufactured on Demand ("MOD") DVDs vary from traditional DVDs pressed in bulk at a duplicator. They are more akin to a DVD-R burned on a home computer. In fact, you may see them referred to as DVD-R.
They are 100% legitimate, being produced by the rights holders of the movie or TV program. It's just that the supplier doesn't foresee a massive interest in the title to warrant the cost of producing, distributing and warehousing it by normal means. They are simply manufactured as demand warrants, thus the MOD name. The are playable on most, if not all, DVD players.
For the most part they have the normal packaging and labels you'd expect. On the other hand, you probably won't get special features common with regular DVDs. These are mostly bare-bones releases.
These MOD releases are available at select internet retailers such as Amazon and Warner Archive.