A resource pack is a collection of images and other files which determine how the game looks. Here’s an example:
Resource pack creators use image painting and editing software like GIMP or Photoshop to digitally draw and paint these images (i. e. they mostly do not use a camera, but start with a completely white or transparent image) and thus the creators are (digital) artists.
Resource packs are not mods. Resource packs only change the look of the game, they do not change the game mechanics, e. g. you can’t add new blocks using a resource pack.