A social enterprise is not a thing, it's a way of operating.
Social enterprises can exist as nonprofits, businesses, advocacy groups, and even governmental organizations.
Simply stated, it's a way of creating a sustainable impact using a market-driven approach. As notably, social enterprises force the entire system they operate within to become more sustainable in the process.
Most often, you'll hear the term "social enterprise" in conjuction with "social good startups" or "benefit corporations", but they are so much more.