Invitation letter Phd from Sweden
They want to meet you face-to-face. This is always a good sign - it means they're seriously considering your candidacy. It doesn't mean they're offering you a position, but it does mean you're one of at most a handful of candidates they have.

If you aren't busy with something else during that period, you should probably go. You get to meet them face-to-face too. You get to see the atmosphere in their department firsthand. You get to experience the city, a sense of how expensive it will be to live there, check out potential places to stay, and so on. If they're covering travel &amp; accommodation you won't even have to pay much to go. Worst case scenario, you could treat it as a holiday in Sweden.