Some retcons are very simple and can involve someone 'retconning' his or her last pose because a previous pose was missed or misinterpreted. So if someone says "retcon that", it's not a big deal, and is likely just a quick adjustment of something due to the hazards of working in a text based environment subject to lag and misunderstandings.

It is also technically a 'retcon' when you rewrite your feature character's history to exclude an embarrassing moment in comic history or some discontinuity. Retcons can also include adding things to a pre-existing history. These retcons are usually dealt with during the process of character approval. If you are already IC and want to retcon something like this, you need to work with the roleplay and Continuity departments, and NFC department if you are an NFC, to get it approved.