Ubercart is an open-source(link is external) e-commerce solution which fully integrates with Backdrop CMS(link is external), a leading open-source content management system. Backdrop CMS was forked from Drupal 7.x(link is external) when its end-of-life was announced and builds on the popularity and ease of use of D7; now with more features built into core and easier to use and maintain.
See "What is Ubercart?" for a more detailed explanation.