It's based on Into the Odd (Chris McDowell) and Knave (Ben Milton). Cairn has just three stats, STR, DEX, and WIL. It has hit points and damage rolls, but it is a roll under stat to make saves. There are no "to hit" rolls, only damage rolls. Damage is aligned to earlier dnd rules (B/X). When I first thought about the not rolling to hit, I thought that would be weird. Actually, in play it is working really well. Damage is taken from a character's hp and then from STR stats. Once STR stat is damaged, player must make a STR save each time he loses STR. Fail = on the ground and dying unless aid.
As I get older, I am drawn more and more to rules lite games. Cairn is very easy to learn...only about four pages of rules...but it has the ability to add crunch where I need it to do so. I like to tinker with house rules and I find Cairn can allow me to do what every I need. OSR monster stats are easily converted and playing modules like Keep on the Borderlands would be fairly simple to do on the fly.
You should check it out, it is free at Drivethrurpg and also the srd and other resources are available at https://cairnrpg.com/
It doesn't have to be only forests. It's just the author's preference for his game. Currently I am working on a hex crawl, open table game using Owlbear Rodeo.