What does the Yoke card mean when it says "Plow"?
The Yoke is talking about the other minor improvements with "Plow" in the name: the Swing Plow, Stump-Jump Plow, Riding Plow, etc. So, for each one that's been built (and is still in play) at the time you build the Yoke, you can immediately plow one field.

If nobody has played any of the Plows/Ploughs/Harrow, then it's not useful. It's cheap to build (as long as you have cattle), but it's also not very powerful.