The Description field is used to specify a more detailed description of the task.

If this is a billable task, this will be the text included for the task in the invoice. If this is not a billable task, then this is optional and is only for your reference.

You can configure this field to automatically populate with template text by configuring the Template Text field in the Configuration / Activities tab.

If this is a billable entry, then there will be a paste icon that will copy the text from the Title and paste it into the Description.