With this product-specific custom field you enter a short description for the corresponding download. It is displayed in addition to the title next to the download link on the product page.
Each of the three downloads (Download 1–3) has its own description field.
You can find the "Download 1 – Description" custom field in the respective product:
Tab "Specifications" ⇒ Block "Custom fields" ⇒ Tab (Set) "Downloads (... Pro)"
Type: Text
Technical name: twt_{theme}_{edition}_custom_field__product__download_1_description