What it is
Logos is Greek for “word,” but for Heraclitus it meant much more: the everlasting principle through which all things are one, the rational order of the world that experience can discover. It was also a moral law; human laws draw their force from it.
The Stoics built their physics on that idea, and later the Gospel of John opened with it: “In the beginning was the Word.”
How it shows up for me
I’m an atheist, and the idea still moves me: that there’s an order to things you can find by paying attention. I once asked, half joking, “Am I logos now?” No. But I can pay attention to it.