The idea is that having a desire for things that don't happen is sure to cause you unhappiness. "Wish" is maybe a bit of a bad term to use there, but it also works- don't hope for things to happen, just accept what happens.

You can still work to make things happen, of course, but don't put any emotional investment into one result. Maybe you try to make your favorite dinner and get it. Great. Maybe you try to make it and the stove breaks and you can't. Okay. If you focus on the difference between the thing you wanted and the thing that happened, you'll just make yourself miserable.