What to expect from an HR interview for a postdoc position?
If your technical interviews went well, I would bet the HR meeting will focus on a combination of the following:


Verifying that you meet formal job requirements. They need to make sure they can actually employ you; you are authorized to work in the country (e.g., are a citizen or have/can apply for an appropriate work permit). They may ask you to confirm that you have a PhD, a common requirement for a postdoc position. Depending on how it is funded, there may be more specific requirements, like having a degree in a certain field or conferred within a certain time period. If you will need some sort of background check, they will go over that process too.
Selling you on the job. If they offer you a job, they want you to take it! They may use this meeting to describe your (potential) position and how you would fit into the organization. There might be a specific discussion of the offer you'd be receiving (e.g., salary, vacation, benefits) or just a more general pitch about how this is a great place to work, etc. Some organizations do this even before the final candidate is actually selected.
Checking for fit/compatibility/etc. Pretty self-explanatory. The impressions of the people you with whom you would work should carry more weight, but nevertheless....


In any case, I would take it as a promising sign that you've been asked back. Good luck!