I thought 'a merge' was the verb 'to go', and to walk somewhere was 'a merge pe jos' (to go by ground). To walk is 'a plimba'. So why is the correct translation for this given as 'a merge'?

If I am not mistaken both merge and plimba can be used to mean walk, while plimba is more literal.

I am in Romania studying right now and using what we learn. Yes a merge is "to go". they say "pe jos" when talking about walking somewhere but I am also realizing some of the expressions depend on what part of Romania you are in.