From the article:
What the bridge does when it is woken up
At rest, the bridge does nothing. The binary does not run until a browser extension with one of the three listed IDs calls it. So on my machine, right now, nothing is happening. That is the one argument Anthropic will try to hide behind. Let me cut through it in advance.
When the paired extension is present and the bridge is activated, it exposes browser automation capabilities to whatever agentic process Claude is running. Anthropic describe those capabilities in their own public documentation.