An invitation from Frontiers (Frontiers Research Topics)
As far as I can tell, a "Research Topic" is a collection of papers on a specialized topic.  It's basically a special issue of one of their journals, although it's presented slightly differently, and they charge publication fees for the papers.

What really turns me off is the publisher's spamming practices.  I've had exactly the same experience you have: they send what's pretty obviously a form letter (populated with your name and one publication title/journal, seemingly randomly selected) and then some months later send a passive-aggressive reminder asking for a reply.  Maybe this is a coincidence, but I also got a follow-up message from them on February 12, so perhaps they sent out a whole wave of them on that day.  In any case, the fact that they feel they have to advertise by spamming makes me suspicious, and the reminder messages are irritating.