The high-resolution designed resume is a Photoshop / Sketch version that comes with a heftier file size. Highly recommended because of the pixel clarity.
The medium-resolution designed resume is a Photoshop / Sketch version that has been optimized to be smaller in size, but by lowering the quality of some elements.
The low-resolution traditional resume is a Word / Pages version. The no-frills version. Not recommended, but it'll do.