reverse seo can refer to 2 concepts: first, it is like reverse engineering. You analyse the top 10 position seo metrics to determine what the search engine algo likes about these sites. The goal after that is to mimic it so you can also rank better. Another reverse seo, would be to do negative seo to a competitor website.
Camay just summed up the concept well. You look at your competition and then build your website keeping the key terms in mind and eventually try to beat them with your own techniques.
Negative seo against the competitor website. The best example of this would be to make use of xrumer not just for your website, but against your competitors website. In this way Google would think that their website is spamming a lot as such debarring them from the search engine