Use this phrase when you don't think you would usually do something, but you can think of some reasons that you would do it. For example:
I wouldn't be able to live in a big city unless I was rich enough to afford a huge apartment with a big outdoor patio.
So the meaning of the example at top is: "If a place is really hard to get around in as a foreigner, I might join a tour. In other situations, I wouldn't join one."
Notice that in the second half, the verb is past tense:
unless it was somewhere that's...
unless I was rich enough...
This is common in spoken English. For formal written English, you're supposed to use the subjunctive tense:
I personally wouldn't sign up for a tour unless it were in a country that's difficult to navigate as a foreigner.