In the context of the construction of a painting, the ground is the primer that’s applied to the canvas or panel. In Painting 1, we use an acrylic gesso (polymer emulsion). In Painting 2, we use a variety of grounds, such as real hide glue (rabbitskin) and chalk gesso, and also casein gesso.
You should choose a ground that’s compatible with the media you’ll be using. For example, do not use an oil based ground if you’ll be painting in acrylics or watercolors; they won’t adhere. If your technique depends on wiping paint away in a reductive manner, you should go with a slicker surface and perhaps one that’s less absorbent. The ground is foundational to your painting; these choices matter.
In the context of basic design, the ground refers to the background or surround, as in figure/ground relationship.