What is the concept/goal of Journey?

Solution 1:

From Thatgamecompany's website:

"Faced with rolling sand dunes, age-old ruins, caves and howling winds, your passage will not be an easy one. The goal is to get to the mountaintop, but the experience is discovering who you are, what this place is, and what is your purpose."

http://thatgamecompany.com/games/journey/

So it appears the goal of the game is to figure out who you are and why you are there.

Solution 2:

The game is intentionally abstract and mysterious. You're meant to have a sense of wonderment and only start to figure out what's going on as you play. To understand the concept beforehand would be a spoiler.

The goal is to find out what's going on, and simply enjoy the experience of the Journey (ideally with other travelers).

However, it appears clear from the hieroglyphs that you're part of an old civilization. At first you see evidence of their vast cities, and then you see evidence of war, those cyclops snake things.
The ending and final achievement hint at "rebirth" as well, so presumably there's some sort of salvation at the mountaintop, possibly even a repeat of the events that occur before the game. Alternately, perhaps you froze to death on the mountain and the ending was a mirage/hallucination.