Sync was a popular 3rd party app for Reddit. With the change in Reddit TOS that prevents 3rd party apps from accessing the site the Dev is making a new app with the same name for Lemmy
It’s used to be Sync for Reddit, which was one of the biggest android Reddit apps. The dev is now building a version for Lemmy. This was posted in the sync for Lemmy community, so context for the people in this community is already there, I assume you’re coming from All so it’s more on you to figure out what it is than for the dev to post about what it is again when the people in this community that the post is meant for already know