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Some Emacs buffers display lists of various sorts. These include lists of files, of buffers, of possible completions, of matches for a pattern, and so on.
Since yanking text into these buffers is not very useful, most of them define Mouse-2 specially, as a command to use or view the item you click on.
For example, if you click Mouse-2 on a file name in a Dired buffer, you visit that file. If you click Mouse-2 on an error message in the `*Compilation*' buffer, you go to the source code for that error message. If you click Mouse-2 on a completion in the `*Completions*' buffer, you choose that completion.
You can usually tell when Mouse-2 has this special sort of meaning because the sensitive text highlights when you move the mouse over it.