Not op but I assume they mean: a main web server is scheduling large jobs in the background by publishing an event to whatever pub/sub system you want to use (there are many!), lambdas can be subscribed to this system so they trigger off these events and publish a corresponding event back when they're done that the web app is subscribed to etc.

For an example, you can look at how node interacts with OpenCV.

The JS scripts together the compute heavy CV alogrithms, and you can do similar by hosting your computer heavy things behind a service call, while keeping the orchestration parts in node