This feature switches from Lightning to HTML upload, enabling 100s of files to be uploaded at once instead of the 10-25 file limit in Lightning. Note that Lightning upload remains part of the default license.
If you would like to learn more about this feature, please visit the following link: Mass File Upload: Additional Feature - HTML File Upload
Demo Video: https://www.youtube.com/watch?v=uGToAiIrqq4&list=PLXqeMW5vDpMA_VqPcBieEV7ZvDgkVYVGv&index=3
PS: View Mass File Upload, Salesforce file upload app on Salesforce AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=82cf236b-1ee8-4ea7-aebf-fd0c89fb93bd