This article contains spoilers for Star Trek: Picard season 2, episode 2.
Star Trek: The Next Generation episode "Yesterday's Enterprise," and it serves as a clue to the model of time travel the show is adopting. Time travel is a common science-fiction trope, but it's actually very difficult for any franchise to handle it consistently. Because time travel is largely theoretical, writers understandably tend to change their model of temporal mechanics based on the dictates of the plot. It's pretty much impossible to established a fixed model of time travel for Star Trek, where there have been countless different writers and showrunners, and there's been real consistency in of time travel at all.
Star Trek: Picard season 2 is the latest time travel story. According to the Borg Queen, Q has changed the timeline back in the year 2024. Rather than create a branched timeline (as in the Marvel Cinematic Universe), this has instead rewritten history, creating a branched timeline in which a brutal and warlike human Confederation has swept across the galaxy, conquering other races and wiping out all opponents. Q has protected Jean-Luc Picard and his closest friends from the changes, and they naturally intend to travel back to 2024 to correct history. Naturally, Q couldn't resist an opening conversation with Jean-Luc Picard - and he made an amusing quip, describing Picard's attitude as being very "Yesterday's Enterprise." On the face of it, this seems to be simply an amusing Easter egg to Star Trek: The Next Generation, but it's actually quite important.
"Yesterday's Enterprise" was a classic Star Trek: The Next Generation time travel episode. It introduced a rewritten present day, with history changed because the USS Enterprise-C had gotten caught in a temporal anomaly and transported to the future; its removal from the timeline meant it had not been present to defend a Klingon outpost from Romulan attackers, a key event in establishing peace between the Klingons and the Federation. In of temporal mechanics, "Yesterday's Enterprise" postulates a single consistent timeline, but one which can be altered by the presence or absence of new factors due to time travel. It's the exact same model of time travel being used in Star Trek: Picard season 2.
There are, of course, crucial differences between Star Trek: Picard and Star Trek: The Next Generation's "Yesterday's Enterprise." In the classic episode, the USS Enterprise-C was transported from the past to the present, causing the disruption; the timeline was corrected when the ship was returned to its own time, and interestingly subtle details seemed to suggest the time travel had always been part of the timeline. In contrast, in Star Trek: Picard it's likely Q has introduced something from the future into the year 2024, and it needs to be managed or removed. So this particular example of time travel will be very different, even if the time travel theories being applied are the same.
It will be interesting to see whether the outcome will be similar to "Yesterday's Enterprise." If that is indeed the case, then when Star Trek: Picard season 2 is over the actions of Q and Picard's crew will have always been part of history, a secret reason why the timeline took its familiar shape. This may even be the real reason Q has arranged this time travel escapade - to teach Jean-Luc Picard something about his past. The past, of course, is the key to the present - meaning these may well result in changes in Jean-Luc's understanding of his personal world in Star Trek: Picard season 2.
New episodes of Star Trek: Picard air Thursdays on Paramount+