[the beginnings of an answer]
Put simply, photographs are made of light. In good light, today's digital cameras make it difficult to take a technically poor photo. Sometimes, though, photos unavoidably come out dark or grey because the subject was in shadow, or it was getting late; worst of all, a cloud appeared overhead and followed you round while the distant landscape was bathed in sunshine. There are ways of making these photos reveal their subjects better.
This isn't a highly technical answer. It refers to features of commonly-used image editing or image enhancement software.
[to be continued]