If you think of Durable Functions as a workflow, then the activity trigger function can be treated a workflow step that takes some kind of optional input, performs some functionality, and returns an optional output. It's one of the core concepts of Azure Durable Functions.
You can learn more about the Activity Trigger at https://docs.microsoft.com/en-us/azure/azure-functions/durable-functions-types-features-overview.
Before we start creating the activity trigger function, let's first build the dependency functions.