In RPG in a Box, the term quick script refers to an ad hoc script that is written for a specific one-time use. This is opposed to a visual script created in the Script Editor which is saved as a standalone resource. An example usage would be for linking one map to another, as the script would be specific to that one scenario and not need to be reused anywhere else. See Script Syntax for details around the required syntax.
Demonstration of a quick script being created in the Quick Script Builder:
Marker icon in the Map Editor indicating that the tile has a quick script attached: