A retcon is something that as a viewer of the TV show, you notice as being different.
For example, if the actor that played Apollo in the first episode was replaced by a different actor for the rest of the series, obviously the characters on the show would accept that as normal (they wouldn't be saying "Wow, Apollo, you look different!") because they're just actors too. But to us, watching the show, we realise that they changed something and pretended it was like that all along.
So changing the design of the Cylon Raider in the 4th season might be a 'retcon', because the effects team changed. Or, it might be a real in-universe reason, such as the Cylons manufacturing an improved fighter.