Insider selling is selling activities undertaken exclusively by persons considered "insiders", people who have access to non-public, confidential information, typically people who are CEO's, company officials and etc. whereas secondary follow-on offering is the reselling of existing shares to the public by current shareholders who may or may not be insiders.
Think it might be a language issue and that they might mean insiders selling - not the stupidly illegal insider trading. Insiders selling whether on IPO, throughout their tenure including after secondary offerings. If I am assuming correctly, its hair-splitting to determine other than on a case-by-case what it all means.
I hope I do not come out as being rude, but for such straight forward, I dare to say questions about theory or terminology, I find investopedia.com to be extremely useful. : ) For upgrading on one's knowledge and diving deeper, for more practical questions, forums are well suited.