Yak shaving describes how when setting out to do something, you found you had to first do something else, which needed you to finish this other thing, and so on until you found yourself shaving a yak, or equally unrelated activity, to do the first thing you set out to do.
It's pretty common in life — tidying your room? — and particularly resonates in software development where when tackling one thing you find yourself fixing something else, which needs you to fix something else, and so on. Here's the ever brilliant xkcd on fixing problems.
The first I found of yak shaving was a short article on productivity in the first issue of Make magazine by Danny O'Brien and Merlin Mann.
Yak in the style of Alison Green and Adam Stower's excellent children's book, What can you Stack on the Back of a Yak?