Data WA's Data usability rating is based on the 5-star open data scheme suggested by Tim Berners-Lee, the inventor of the World Wide Web. The rating indicates how reusable and linked the data is.
1 star ★ data is available in some format, under an open licence (such as Creative Commons).
2 stars ★★ data is available in a structured format, such as Microsoft Excel file format (.xls).
3 stars ★★★ data is available in a non-proprietary structured format, such as Comma-separated values (.csv).
4 stars ★★★★ data follows W3C standards, like using RDF and employing URIs.
5 stars ★★★★★ all of the other requirements, plus links to other Linked Open Data sources.
If you’d like to learn more about linked data, Wikipedia - Linked data is a great starting point. A 2009 TED talk by Tim Berners-Lee, The next web, is also a great introduction to linked data concepts.