Mushishi is about these weird, ethereal beings called mushi, they're basically the most primitive form of life you can imagine, existing somewhere between living and not living. They're not good or bad, just... there. People have always been both scared of them and fascinated by them because mushi can mess with your life in ways you can't really explain.
The show follows this guy named Ginko who wanders around investigating strange things that happen when people run into mushi. He's got this deep understanding of how they work, almost like a shaman, and he helps people deal with whatever's going on, trying to figure out what the mushi are doing and why.
Each episode tells its own story, but they all touch on similar ideas: how humans and nature have to live together, the invisible forces affecting our lives, and finding that balance between wanting to understand mysterious things and knowing when to just leave them alone. It's a really contemplative show, more about quiet moments and philosophical questions than big dramatic conflicts.