The graph shows that, as teh time increases, the altitude is decreasing in the first period and increasing in the last.
It may represent that the hiker is crossing from one mountain to the other, going through a valley, where the altitude is less.
It can also represents, more probably, that he returns to a base, in a low-altitude place, and go on again up in the same mountain. It seems more probable as the final altitude is the same as when t=0.
We can draw it as: