Lineland is where the flip wizard Bestouvius is and the first chapter in the game. It is where Mario is granted the ability to flip between dimensions. It has a few sections from 1-1.
Mount Lineland is a mountain located beyond Lineland Road, and consists the village of Yold Town. It is where Mario gets his second Pixl, Thoreau.
Yold Desert is a desert located near Yold Ruins, and where O'Chunks is fought. It is an infamous level because of its hard puzzles.
Yold Ruins is a mini-dungeon in Chapter 1-4 and it needs to be beaten to defeat the game. It has a few challenging puzzles and hidden secrets. Fracktail is fought at the end, and after Mario defeats him, he will get the second Pure Heart.