BU Content refers to content that belongs to a fictional universe Big Hit Entertainment is creating as part of BTS' body of work (BU = BTS Universe), and the characters in the BU borrow the names and age dynamic of the BTS members in real life. However, not everything BTS releases is part of this universe, so relevant content is marked with either the BU Logo, or "BU Content certified by Big Hit Entertainment" at the end of Youtube video descriptions.
Chronologically, BU content dates back as far as mid-2015 with I Need U, but several videos were retroactively marked as part of the BU in mid-2017, when the BU was officially established in time with BTS' rebranding and the release of the Wings Concept Book, which is where the BU Logo first appeared.
In fandom circles, the BU is sometimes still referred to as HYYH, which comes from the videos I Need U, Prologue, and Run. These videos were part of the album trilogy The Most Beautiful Moment in Life (kor. 화양연화, Hwa Yang Yeon Hwa = HYYH), and are now widely understood as the foundation of the BU, but fandom already recognised they belonged to the same fictional universe before the official introduction of the BU, hence the fanmade name HYYH.
A general warning: the BU story deals with potentially triggering themes like suicide, abuse, accidents, and mental illness. None of these themes are treated lightly or trivialised, but if you are generally sensitive to these topics, please proceed with caution! Specifically the webtoon is very graphic. In the timeline there is an option to display trigger warnings.
* Due to a glitch, some browsers don't display the "BU Content certified by Big Hit Entertainment" in the YouTube description, but it's there
** For the sake of brevity this list only contains the longest/final versions of repeated content (e.g. the Japanese Blood Sweat & Tears teaser is marked as BU Content, but only contains scenes from the MV, so only the MV is listed)
Additionally, the Naver blog Testesso (translation) contains post written by the owner of the fictional "Smeraldo" flower shop, a shop Seokjin encounters in the story (he also tweeted about the Smeraldo flower a couple of days before the Love Yourself era was introduced). The blog posts describe the history of the flower and the events leading up to the shop's opening. While none of the BU characters are mentioned by name, there are hints that the customer the owner talks about in the blog posts is Seokjin. Posts have also been added and/or changed a few times when BTS was gearing up to release a new album. While this might be subject to change, so far the blog doesn't really contribute to the plot itself, but merely provides some background information and context.
Some BU Content makes very clear references to the BTS Begins concert VCR, and while we have no way of knowing if it's officially part of the BU because Big Hit cannot retroactively add a BU marker to this, it's safe to assume that it's at least partially relevant.
Song lyrics have never directly contributed to the BU storyline so far, if at all they only reflect the general mood or topic of the video content.
The timeline is a chronological list of events in the BU, with information primarily taken from the Highlight Reel and HYYH the Notes (short: the Notes), which are part of the physical package of the Love Yourself album trilogy, one set for each album plus one additional teaser note uploaded on the official Big Hit Twitter before each album release.
The purpose of this list is to structure and connect what we canonically know about the BU, without any further interpretations of the story apart from a few conclusory notes about what's hidden directly in the text. It's meant to be an aid for those who want to understand what's going on, or want to draw their own conclusions about what's happening below the surface.
Prior to the release of the Notes and Highlight Reel (i.e. the start of the Love Yourself era) there was no information on which events happened when in the universe, and while the storyline is becoming clearer, there is still a lot of (video) content that currently can't be placed into any timeline. Seokjin time travels in an attempt to save the other characters, but for now it remains unconfirmed if the Notes describe events from different versions of the universe as a result of Seokjin's actions, or if they all belong to the same one (with the exception of Seokjin's notes from Aug 8, Year 22, where we already know there are two different versions thanks to the Highlight Reel).
Currently, the timeline doesn't include any events from the SAVE ME webtoon, since it's still being released and also shows multiple versions of how the story can play out on the same days, which would blow up the timeline considerably yet not contribute much to the understanding of the chronological* events. It's probably best to read the webtoon in its entirety, and use the timeline as a guide, since the webtoon makes quite a few references to scenes from the Notes. As of chapter 10, none of the events in the Notes are ones that directly cause Seokjin's attempts to save everyone to fail.
* 'Chronological' in this case referring to events from the universe that Seokjin no longer attempts to change.
All new notes contained in the HYYH the Notes 1 book will be added as soon as possible after the release (March 5).