Necroposting is when you post something on an old thread. Example: Let's say a thread was posted in December 1st, 2014. And nobody posted on there. Then let's say you decided to post on there at the date of February 9th, 2015. That would be considered as Necroposting. For a more understanding explanation: It's if you post on a thread that hasn't been touched in 1-2months.
What CREEPERBOT said. It's posting on a "dead" thread in attempt to revive it. And general rule of thumb is if the last reply was over 3 weeks ago, posting a new comment would be necroposting. Necromancy is the practice of bringing things back from the dead.
