It means a friendly invitation for a quick chat while having a coffee or a mineral water or a softdrink or even a beer! It’s an informal friendly way to invite someone we just cross over on the street, for instance, and with whom we’d like to update our conversation.
It can also mean that we are inviting for half an hour/an hour of informal conversation with someone we must talk to but we don’t want to invite to the office or at home. The important thing is that it is brief. Oh, and it shouldn’t be an expensive moment, either.
Lets have a coffee one of these days?