Contributing to open-source projects is a goal of many programmers. Issues tagged with "good-first-issue" is one way to find something to work on.
When newcomers (people who never contributed to the project) browse issues in a repo, GitHub will hit them with a banner "If you're ready to tackle some open issues, we've collected some good first issues for you.", which will take you to an issues page filtered with the label good-first-issue.
Think of this page as a landing page for your repo to newcomers, since it's pretty much the first thing someone will look for when they're looking to contribute (most repos will also link this page in CONTRIBUTING.md). But I've found this page to be downright helpful in most cases. Usually it's filled with issues which fall into one of these buckets:
The issue is written as if the newcomer is supposed to understand a lot of context. Newcomer has asked a question, no one has answered.