In order for your photos to appear in Fresh (https://500px.com/fresh) upon upload via Lightroom minimum requirements of Title, Description and at least 3 Keywords should be added. Here is a step by step guide to add the necessary metadata to your photos:
Locate Metadata tab in the right-hand panel
Select 500px preset from the drop down list
Add Title, Description and Category to your photos
In the Keywording tab add at least 3 relevant tags to describe each photo
(Marketplace submissions require a minimum of 5 keywords!)
You are ready to Publish your Photo!!!