It is the complete collection of all 5th Edition SRD monsters converted to play with the Shadowdark RPG system. Each of these monsters was adapted to act close to like how they do in 5th Edition, but making them work in Shadowdark.
In some cases, the levels are lower than their 5e versions because Shadowdark tends to play at lower levels. Also, while some of the monsters are in the Shadowdark GM rules, the ones in this set of cards are different, they were all re-adapted.
Shadowdark's system makes it very easy to use monster cards, as the monster card text is very short and doesn't need a lot of information.