To understand what "mode of action" means with regard to drugs, take a look at the following explanation from Encarta:

http://encarta.msn.com/media_461540254/Drug_Mode_of_Action.h...

Drug Mode of Action
Certain drugs work by interacting with receptors, special sites on the surface of body cells. Drugs may bind to a specific receptor, possibly preventing naturally occurring chemicals from binding to the receptor. In so doing, if a drug enhances cell activity, it is called an agonist; if it blocks cell activity, it is called an antagonist.

وتسميه العرب آلية الفعل أو آلية العمل

You can google the term around to see how it is used in actual context.