`wal` is a script that takes an image (or a directory of images), generates a colorscheme (using `imagemagick`) and then changes all of your open terminal's colors to the new colorscheme on the fly. `wal` then caches each generated colorscheme so that cycling through wallpapers while changing colorschemes is instantaneous.
`wal` also merges the new colorscheme into the Xresources database so that programs on your system such as `Rofi` or `i3` use the new colors automatically. `wal` finally exports the colors into various formats so that you can use the colors in webpages, scripts, other programs etc.
For more info, check out the [Wiki](https://github.com/dylanaraps/pywal/wiki).