A coffin handle is a handle shape whose outline widens from the butt, reaches its widest point around the middle, then narrows again toward the blade, giving it the elongated hexagonal silhouette of a classic coffin. It is a traditional pattern found on bowies and on some folding knives.
Straight lines rather than curves, with angled corners where the width changes. Seen flat on, the outline is unmistakable, which is where the name comes from.
It appears most on traditional and bowie-style fixed blades, and it turns up on folders as a design reference to those patterns.
The widest section fills the palm and the narrowing toward the blade gives your index finger somewhere to index against. Because the sides are flat, it also resists rolling in the hand, which is a real advantage on a large blade.
The flat panels are a natural place for inlays, engraving or contrasting scale material, which is part of why the pattern persists.
It is a preference. Contoured handles fit more hands more comfortably; angular ones index better and look distinctive. On a large knife the anti-roll property is worth something.
The name refers only to the outline. It carries no other meaning.