A "timeline" is a list of successive dates and/or events. For example: the date when the first successful spaceship was launched, the date when the first astronaut went into space, the date when when the first man walked on the moon. The timeline can be more detailed by including the first American space flight, the first Soviet space flight, the first flight by a woman astronaut, etc. A timeline is as detailed or as simple as one wants. The article is talking about a timeline of experiences that are emotionally important to a person. The author refers to it as "concentrated" because every person experiences a wide range of emotions every day but we don't remember every emotion. For example, regarding a trip to a foreign city for the first time, we might remember the lovely hotel room we stayed in, an especially charming or unusual restaurant we ate at, and/or a museum exhibit that really impressed us. We are less likely to remember that our flight was delayed 30 minutes at the airport, whom we talked to while on line to use the toilet, what we ate on the flight, and how the waitress treated us in an unremarkable restaurant where we quickly ate lunch. That is, we don't remember every action, event and emotion during a vacation. Instead, we remember what is emotionally significant to us. Often it is something that we really enjoyed but I would add that it can also be something particularly unpleasant or highly unusual. For example, a person would probably remember if he or she lost his/her wallet. The author is thus saying that our memory of what we experienced which was originally very detailed and concentrated, over time becomes a short list of only the most important -- and often most pleasurable -- experiences.