Collision Override is a property that can be configured for tiles from the Model Properties panel. It affects two different aspects of how the player and their character behaves with a particular tile, specifically how the player character will follow the terrain as they walk across it and the boundaries for determining if the tile has been clicked on.

See Terrain Following for more information about each type.

Overriding a tile's collision also affects the boundaries of where the player is able to click in order to walk to that tile. This can be beneficial in cases where a tile's shape potentially makes it difficult to be easily clicked on with the cursor (for example, a narrow bridge or walkway) and you'd like to provide a larger clickable area.

As shown in the example below, the wooden board on the left has a fairly small clickable area due to its narrowness. The wooden board on the right has been assigned a collision override of a full 16×16 tile, therefore the player will not have to be as precise when attempting to click on it.