Contrast refers to the amount of black and white in an image, Low 
contrast means an images has very little Black blacks and equaly few 
White Whites, and can look washed out / Flat.
High Contrast images 
meanwhile have a lot of black blacks and blown out whites which can give
 a lot of punch to an image but also means it may lose important 
details. 
There are several ways to improve or correct the contrast 
in your photos and here is how to do it with the free photo editing 
software called Gimp.