Legacy code is basically code that is difficult to work with but we have to change. One of the reasons its difficult to work with is probably that the design is poor and it lacks both modularity and automated tests. Adding tests to this kind of code is often a chicken-and-egg problem. You can’t add tests until the code is redesigned to be more modular, and you aren’t confident to change the code to make it more modular until you have some tests.
The ‘Legacy Code’ theme is closely related to both Refactoring and Test Design, but I tend to put it in its own category since there are techniques for handling legacy code that you wouldn’t need in other situations. See also the theme of Approval testing in legacy code