In modern English apple pie, apple slices are layered into a dome shape to compensate for shrinkage and prevent a saggy middle.
Apples lose water and collapse as they bake. If the slices are packed into a flat, level layer, the cooked filling can settle noticeably below the rim while the pastry crust remains fixed in place. Building the apples higher in the center gives the filling room to descend while preserving a fuller shape after baking.
This technique is associated with English-style apple pie, especially pies made with thick layers of sweetened cooking apples such as Bramley apples. The mound is covered with shortcrust pastry, often with butter or lard, and baked until the apples are tender and the crust is browned.
A dome is different from a lattice pattern. Lattice pastry describes the arrangement of strips across the top, while the dome describes the shape of the apple filling underneath. The method also differs from simply spreading the apples into a flat layer, which is more likely to leave a hollow center.