Reaction means when you enjoy or suffer. That is called reaction. Inaction means when there is no result on your account.
Just like you are working on account of the state. The state orders you to fight so you are fighting, you are killing so many men. There is no reaction. But without state's order if you kill one man, immediately becomes a murderer. There is reaction immediately. This is very simple to understand. Similarly, if you act on the supreme order there is no reaction and if you act on your own account there will be reaction. Own account means whatever you do, either you suffer or you enjoy. But if you want to be inactive, neither suffering nor enjoying, in the neutral state, that is required, that is Kṛṣṇa consciousness. Go on.