I get at least one of these each day. The idea is that I pay them money, and they make sure that my web site is "registered" with the search engines. They don't say which ones, "registered" is meaningless and they won't actually do anything.
It's a very old scam, I've been seeing this for 20 years now. But the fact that I'm still seeing it, means that it must work on at least some people.
The truth is, you don't have to "register" with search engines. They find you and list you.
Search Engine Optimisation, however, really is a thing, although I suspect that most of the people spamming me to offer it, don't really know much and will be poor value for money. If you want to make your web site more noticeable to search engines, there are some things that you can do - Google and you'll find a bunch of sites giving good advice. And then you can do it yourself, very easily.