There is something to understand about action, something to understand about wrong action, and something to understand about non-action. The course of action is hard to fathom.
Explanation
Krishna has just promised to explain action (karma) so that Arjuna can be freed from harm. Here he says three things must be understood: right action, wrong or forbidden action (vikarma), and non-action (akarma). He adds that the way action works is deep and subtle. Traditions differ on what akarma means, for example inaction, or action done without attachment to its results.