Link Dumping is the Black Hat art of dropping a link to your sites everywhere that is humanly possible for little to no money. Common link dumps include guestbooks, comment sections, and link exchanges.
An example of link dumping would be to read an article and then in the comments put your url in and keep doing that all over blogs/ forums/ etc.
But commenting and forum posting with links is another method of link building, then how it is black hat seo technique. Will it harm my website ?
Comment posting and forum posting it's not a black hat method but if you are doing so much comment on irrelevant blog or forum then it can be harmful for your site