Chrome is the visual design elements that give users information about or commands to operate on the screen's content (as opposed to being part of that content). These design elements are provided by the underlying system — whether it be an operating system, a website, or an application — and surround the user's data.
Not coincidentally, "Chrome" is also the name of Google's web browser, though I don't use the term in that sense here.
I don't know who came up with the term "chrome," but it was likely a visual analogy with the use of metal chrome on big American cars during the 1950s: the car body (where you sit) was surrounded by shiny chrome on the bumpers, tail fins, and the like.
Similarly, in most modern GUIs, the chrome lives around the edges of the screen, surrounding the middle area, which is dedicated to the user's data.