To specifically answer your question "what should I make of this", they might be asking you to come in for a number of reasons:
- To sign the contract / approval for them to begin the background check
- To have you meet some people to help you be sure of your decision
- To discuss start dates and any other potential situation about your offer face to face rather than over phone/email.
Overall, there is nothing to be worried about - if they have asked you to come in and have already made you an offer, this can only be positive.
Good luck and, again, well done!