Above is the origional cropped image and below is the edited version. to get the colours in the sky brighter and to look nicer i used the levels tool to adjust the colour, brightness and contrast.
1. The histogram is a graph that shows you the amount of pixels of each colour in the picture, and it shows RGB(red, green, blue)
2. An under exposed image is where the image is to dark. An overexposed image is where there is too much light in the photo. a correctly exposed image is where there is the correct amount of light in the photo. also the histogram to the left of the image is spread out all over the graph.
3.A high key image is when the image is mostly made of white and a low key image is where the image is made of mostly black pixels.
4. Levels can improve the the vividness of the colours and make the image less washed out
5. I used the magnetic lasso tool to select the sky to change the levels. you could also use the magic wand tool or the lasso tool.