Transitions and autotiling

Real levels need borders — grass meeting dirt, floor meeting wall. A single tile can not do this, so you make a set of transition tiles: edges (grass-on-the-left, dirt-on-the-right), inner corners, and outer corners. Placed correctly, they make one material flow smoothly into another instead of ending in a hard square edge.

Autotiling is when the editor/engine picks the right transition tile automatically based on a cell’s neighbours. The classic setups are the "blob" (47 tiles) and the simpler "wang"/16-tile sets. You draw the transition pieces once; the tool assembles them for you as you paint.