As of now, the "description" field on idea custom fields is for internal setup reference, and does not present as descriptive help text on an idea portal showing the field.
It would helpful (for any custom field type) to have an option to provide descriptive help text on the field that is customer-facing on the portal.
Now the ideas portal shows your customers a help tooltip next to each custom field that has a description.
As a customization admin in Aha!, go to Settings > Account > Custom fields to enter the description help text for a field. And if a custom field already has a description, you can override it for your portal in Portal settings > Terminology > Custom fields.