[DiscoBahb]

Think of it this way: in something like copper, atoms are arranged in a 3d lattice, and electrons are able to jump (or "tunnel") from copper atom to copper atom freely in any of the x, y or z directions because the lattice looks the same in each direction.

Some more complex materials do not have this symmetry, and the ability of the electrons to tunnel is much stronger in some directions than others. In many known high temperature superconductors electrons are more or less confined to move in a 2d plane (even though a lump of the material is obviously a 3d object) so they are called "quasi-2d".

In other materials they are confined to move in only one direction, which seems to be the case with LK-99