Reddit: What's New is a very simple, yet useful extension for Reddit that highlights new posts. You can also disable the highlight or turn it off if it's not a use to you.

The extension works as follows:

Once installed, you need to enable it in the Reddit app settings.

Open the extension in the app. You can open a post, open the settings, or use the tab icon.

Click on the tab to view a list of the posts that are new since your last visit.

It's a useful add-on that will help you know which subs you've already read.