it is a three dimensional zone in image space where your lens is the sharpest, falling off as you leave the zone
these articles will help you understand what your depth of field actually looks like
https://www.dpreview.com/articles/1351719699/roger-cicala-field-curvature-for-fun-and-profithttps://www.dpreview.com/opinion/7031211310/roger-cicala-field-curvature-pt-2the best photographers on earth always knew this intuitively and shoot manual focus because all lenses have field curvature and autofocus only focuses on-axis in one spot. DSLRs are still preferred by many specifically because it is possible for someone with good enough eyesight to see field curvature in the entire scene.
large format photographers move the lens and film independently to change the shape of their depth of field (so that it runs sideways along the ground, for example) to take photographs that are impossible to replicate on smaller cameras.
