While reading the node.js docs I came across

Clusters of Node.js processes can be used to run multiple instances of Node.js that can distribute workloads among their application threads. When process isolation is not needed, use the worker_threads module instead, which allows running multiple application threads within a single Node.js instance.

I thought of worker threads being isolated, parallel processes already. What exactly is meant with "process isolation" here and how are clusters different from "Multiple threads within a single node.js instance"?