Post Commentary or narration is the second of two subsets of narration for video Let's Plays. It involves narrating the footage of the game at some point after it has been recorded.
Advantages and Disadvantages vs Live Commentary
Post commentary allows for more thought to be put into what is said in a video Let's Play; not necessarily to script the narration entirely but to make relevant commentary which is informative and/or interesting without having to think things up on the spot (and often forget what to say). Post Commentary also allows for more concentration when playing the game for pulling off tricks or shortcuts than what might be possible when dividing attention between the game and what is being said.
Post Commentary can also be useful in some cases for players incapable of playing the game and narrating at the same time, for whatever reason. For example, if one cannot set their PC to record from both the game and microphone properly at the same time.
On the other hand, Post Commentary can remove a lot of the surprise factor and humour of a Let's Play because the narrator already knows what has happened in the video. The simple act of being hit by an enemy unintentionally loses all of its humour value when it doesn't yield a reaction from the player.