The other answers are right, just explaining in simpler language in case anyone needs it:

Codeberg, like Gitlab and Github, is a site for hosting and managing code repositories. These make it easy for many people to collaborate on a software project, review code, keep track of changes and history, keep track of bugs and feature requests, and more.

Here's an example of a (very active) code project to explore: https://codeberg.org/tenacityteam/tenacity (An audio editor based on Audacity)

The most famous code repo management tool, GitHub, was bought by Microsoft a few years ago, so reliable community-run alternatives like Codeberg are increasingly important.

Turn off downvotes so you don’t have to care

It's a thing for people to share git repositories and see the diffs and send changes. Common alternatives are gitlab and github. Codeberg is a fork community maintained after the original owners started to be weird and doing stuff that people didn't like. It's written in Go language. Small and fast for people to deploy themselves and maintain, in complete opposite from the common alternative people used gitlab which is a huge pain to self host and needs enormous resources.