A dieline is a technical template showing exactly where a piece of packaging will be cut, folded and scored — the flat, unfolded blueprint that the printed design gets applied onto before it's assembled into its final 3D shape.
Packaging design has to work around the dieline's constraints, not the other way around — text and imagery need to be placed so they land correctly once the piece is folded, avoiding fold lines and cut edges that would otherwise interrupt the design.
Dielines are typically provided by the packaging manufacturer or a structural packaging designer, and a graphic designer then designs the printed artwork to fit within it precisely.