Explain what "or via queue worker by Image Style Warmer" means

Problem/Motivation

Looking at the configuration page of the module, I'm wondering what

or via queue worker by Image Style Warmer.

exactly means.

Could you maybe add 1-2 sentences of explanation when the warming happens? (What this is triggered by?)

Does it mean on cron run?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes