Notice how the photograph is a rather uniform shade of gray. We can "open up" this image by using the Levels layer.
The Levels window shows the photograph's histogram. The black triangle below the histogram is the photographs' "black point". The white triangle is the photograph's "white point", and the middle gray triangle is the photograph's "mid gray point". Notice how the bulk of the histogram is to the left of midpoint, while the right side is flat. This means that the photograph has predominantly darker shades of gray while the lighter shades of gray are virtually non-existent. We want to adjust the histogram so that it's more evenly spread out. We do this sliding the three triangles. Since the histogram is mostly to the left, we want to move the white triangle in that direction. Click on the white triangle and drag it to the left.
Notice how as the the white point moves left, the midpoint also moves in proportion. The photograph now seems brighter. This is because there are now more pixels to the right of the midpoint than before. The histogram now lies more evenly on either side of the mid point. Here is the resulting photograph:
However, there's a problem. Notice the bright spot in the top left corner. This is called a "blow out". That's when a part of the photograph becomes so bright the detail is lost. This happens because some pixels which used to be gray are now to the right of the white point. These pixels are now pure white. We want to avoid blowouts, so let's modify our levels adjustment. Rather than pushing the white point to the left, let's adjust the midpoint instead.
Here's the result:
The histogram is more evenly distributed. The image is now brighter without blowing out the sky. But the shadows aren't as dark as they used to be. To fix that we'll move the black point to the right.