As Steve Krug explain in his book Don't Make Me Think, "usability really just means making sure that something works well: that a person of average (or even below average) ability and experience can use the thing -- whether it's a Web site, a fighter jet, or a revolving door -- for its intended purpose without getting hopelessly frustrated" (p. 5).
Depending upon the scope of the project, usability can be assessed in different ways. One common approach ("usability heuristics") refers to the use of a checklist to identify potential usability problems. The checklist approach is acceptable for smaller scale projects, and using a checklist is certainly better than nothing, but you should always try to evaluate your work by recruiting real-world users. This is the very best way of understanding the strengths and weaknesses of an interface.
High-end usability testing can involve the use of expensive equipment, one-way mirrors, and advanced recording devices, but this is often overkill. You can achieve similar results by conducting "guerilla usability testing" on a shoestring budget.
To earn this badge, you will engage in "guerilla usability testing" to evaluate an existing web site. . The document that you create as part of this badge is the sort of thing that you could include in your digital portfolio as an example of: (a) your understanding of digital usability, (b) your understanding of how audience research methods can be used in a professional, real-world context, and (c) your abilities as an effective writer and strategic thinker.
It might sound far-fetched, but including something like this in your portfolio could help you land a job with a small-to-medium-sized agency. Many agencies are not able to hire usability specialists as full-time employees or as highly-paid contractors. Savvy potential employers would be thrilled to meet a job candidate who combines usability awareness with other technical skills.