What does the title of first chapter of Attack on Titan (進撃の巨人 Shingeki no Kyojin) mean or signify in the context of manga? To You, 2000 Years From Now (二千年後の君へ Ni-sen nen-go no Kimi e) is the 1st chapter of the 1st volume and the 1st chapter overall of the Attack on Titan manga.



Who is the you?

And what is the significance of bringing up a timeline 2000 years from now?



One thing that is bugging me that the chapter,  volume and manga starts with Eren waking up fro a dream, crying and exclaiming that Mikasa's hair has grown longer?

As put in wiki:


  Meanwhile, Eren Yeager is having a dream about an older version of his close friend, Mikasa, bidding him goodbye. He is promptly woken up by the real Mikasa, who insists that he is been sleeping for a while. As the two return to their village, Eren begins crying, but when Mikasa asks why, he does not give an answer.
This answer contains spoilers.

Please do not read if you are not up to date with the manga (latest chapter being 91).

TL;DR


   The you might be the future inheritor of the 'Attack Titan' power.


Also keep in mind that there is no official answer, thus the correct answer (for now) would be "We don't know". 

However, after discussing on some forums and reading some theories, some hints provide a common direction.


   As we know, Eren obtained the power of the Attack Titan with the Coordinate Titan (also known as the Progenitor Titan) from his father, Grisha Jaeger. Ever since, he has been having certain "visions", or "memories" especially while fighting other titans. In the manga chapter 89, this becomes clear. 


SPOILERS!! Images from manga chapter 89.

(Picture source: Mangastream Disclaimer: I do not own these pictures. They are only used to answer this question more clearly.)



Leading to Hange asking:



Which brings us to the second hint of this chapter. Eren recalls the time he was able to use the Coordinate Titan's power.





Focus on "everything connected".

Which brings us to the culmination of the chapter (and the 'answer' to your question): 

SPOILERS! If you have not read chapters 87 and above, do not read further!





Whose memories are these, I wonder

TO ANSWER YOUR QUESTION


   There exists a memory-timeline that is outside of our perception of time. That is, Eldians who have obtained (at least) one of the 9 original Titan's power, have access to the memory-timeline of all previous (and possibly future) Eldians who obtained the same power!! In simple terms, Eren has the 'Attack Titan' power and the 'Coordinate Titan' power. He is able to access his father's memories, and either alter them or combine them with his own memories. Thus, the you in the first chapter, might be the future inheritor of the 'Attack Titan' power. As stated above, this is all speculation because nothing is revealed yet.