The scripting engine is a new feature of DCS since DCS: A-10C. The engine provides mission designer with access to game world and game objects using scripts. It's possible to change object properties, control object behavior and read object properties. The scripting engine was developed primary to be used for A.I. control. This is the way to create more sophisticated behavior of A.I. units than it possible using ME GUI because you can take in account more factors.
Note: The engine was initially developed for debug purposes. It was not tested properly, not tweaked, not reworked or extended due the requests of the community, so now it may contain bugs and disadvantages.
The document is actual for DCS: World 1.2.6.