A play on words, a mixed metaphor combining "rocket science" and "brain surgery", both of which are colloquially used to refer to something very difficult. Therefore, saying that something “is not rocket surgery” is meant to imply that said thing is rather easy to do, and doesn't require any specialized learning or training.

In any case, it shouldn't be rocket surgery to understand the meaning here by the context alone even if you've never heard the phrases “it's not rocket science” or “it's not brain surgery”, but since we seem to be dealing with people, for lack of a better term, incapable of understanding the extremely simple concept of not browsing sites which might contain content unsuitable for work while at work, maybe it turns out that it is in fact as hard as rocket surgery for you lot, but in that case you should be asking your legal guardians, not random strangers on the internet.