Yeah, my understanding is that it sets all the story quests to "completed", it gives you any titles/mounts/minions you would have earned along the way(but not all the armor and other assorted stuff), and puts you in one one of the grand companies if you arent already(defaults to the city your character started in). It also tags the required dungeons and trials as completed, letting you basically jump right in to the start of the next tier of content. You can open the "completed" tab in your quest journal and read the short synopsis of any of the 500ish quests(i dont feel like looking up the actual number) you skipped. And using "The Unending Journey" interactable furniture item(from an inn room if your house/fc doesnt also have one) you can rewatch the important cutscenes(and many of the unimportant ones, but they arent all on there and this only gives access to cutscenes and not other character interactions).
Basically, if you like(or think you might later) the story i highly advise not buying the story skip potion. Level skip is a personal choice, but experiencing the story second hand is just not the same. And if you take the time leveling takes out of the equation, you can go through the story a lot faster then you might think. Playing something like 10 hours a week(doable by all but the most casual of players) and focusing just on the story, you can clear the base game along with the first expansion and everything leading into and out of both in a roughly 6-8 weeks(if we arent counting leveling it may even be a little less then this). It will be 3+ months before the next major patch, so that timeline would even give you time to get through the newest expansions current level of story. If the patch is late, youd likely even have time to get completely geared up to dive right in on patch 4.2. And that math was for a new player. It looks like you(OP) are already more then halfway through that content. Ultimately the choice is yours.