Pillow is a fork of a library called Python Imaging Library, or PIL. The fork became necessary because the original PIL become under-maintained and eventually lost compatibility with newer Python versions.
At a high level, Pillow lets you take in an image from one of a large number of formats, and then it gives you back a format-agnostic Image object.
Pillow is a de facto standard across major ML libraries, most notably TensorFlow and PyTorch installation.