The 'Trending' sort order is available when using the Trending Sort app and it is an alternative way to sort your products provided by Power Tools app on top of the existing Shopify sorting options.
You can think of a 'Trending' sort collection as a manually sorted collection that Power Tools is constantly updating on your behalf.
While viewing collections in your Power Tools admin a Trending collection will show the sort order as 'Trending'
When viewing collections your Shopify admin, a collection set to use a 'Trending' sort order will show as sorted 'Manually'
The collections which are set to the Trending sort order will be shown as 'Manual' (or 'Featured' for most Shopify themes) at the store front end, while the app will be updating the collection's sort order based on product views internally.
The 'Trending' sort order can't be added as a new option in the default Shopify "Sort By" section in your theme, but we can help to change the text from 'Manual' or 'Featured" to 'Trending'.
If you have any additional questions, please contact support through the live chat in the app, or at firstname.lastname@example.org