I'm sure your dictionary will say that "to tarry" means to delay, to take your time, not to hurry.
(Kind of a poetic word that you don't hear in conversation.)
I've never heard that proverb before, but I imagine that it must mean
take your time in making an important decision (such as getting married).
Don't be in a hurry when you make a decision that will affect your whole life.
Thanks, Biffo. That explains the "wherein", which was puzzling to me.
"Be sure about the place in which you will stay."
(I thought it meant "Be sure you are choosing the right partner",
but "wherein" does mean "in what place".)