A "design proof" is a preliminary version of a design, presented digitally, that allows customers to review and provide feedback as a last and crucial step before it goes into final production, essentially acting as a preview of how the finished product will look, enabling any necessary adjustments to be made before finalizing the design.
Design proofs are not 3D mockups, but the flat dielines/product template that will be fed into production – similar to a mockup. A proof typically details safe areas (the buffer zone where all important text and images should be placed to ensure they won't be accidentally cut off during the trimming process), trim lines (where your design will be cut), and bleed lines (the area of a design that extends beyond the trim line, ensuring a clean edge on the final product by accommodating slight paper movement during trimming and preventing white borders).