karmany akarma yah pasyed
akarmani ca karma yah
sa buddhiman manusyesu
sa yuktah krtsna-karma-krt
One who sees inaction in action, and action in inaction, is intelligent among men, and he is in the transcendental position, although engaged in all sorts of activities.
Commentary by Srila Prabhupada
A person acting in Krishna consciousness is naturally free from the bonds of karma. His activities are all performed for Krishna; therefore he does not enjoy or suffer any of the effects of work. Consequently he is intelligent in human society, even though he is engaged in all sorts of activities for Krishna. Akarma means without reaction to work. The impersonalist ceases fruitive activities out of fear, so that the resultant action may not be a stumbling block on the path of self-realization, but the personalist knows rightly his position as the eternal servitor of the Supreme Personality of Godhead. Therefore he engages himself in the activities of Krishna consciousness. Because everything is done for Krishna, he enjoys only transcendental happiness in the discharge of this service. Those who are engaged in this process are known to be without desire for personal sense gratification. The sense of eternal servitorship to Krishna makes one immune to all sorts of reactionary elements of work.
Commentary by Sri Visvanatha Cakravarti Thakur
This verse gives an understanding of the truth of action and non-action. He is intelligent who sees that persons of pure heart fixed in knowledge like Janaka who do not renounce activity, but rather engage in action, in niskama karma yoga, do not accrue karma (akarma); and who sees that one who does not perform actions, renouncing actions as a sannyasi, being of impure heart without knowledge, though talking profusely of knowledge because of knowledge of scriptures, actually obtains bondage of karma leading to misery. He engages in all activities; he does not renounce action even through instructions or association of those who think themselves knowledgeable, talking a lot about jnana.
The Bhagavatam says:
yas tv asamyata-sad-vargah pracandendriya-sarathih jnana-vairagya-rahitas tri-dandam upajlvati suran atmanam atma-stham nihnute mam ca dharma-ha avipakva-kasayo ‘smad amusmac ca vihiyate
One who has not controlled the six forms of illusion [lust, anger, greed, excitement, false pride and intoxication], whose intelligence, the leader of the senses, is extremely attached to material things, who is bereft of knowledge and detachment, who adopts the sannyasa order of life to make a living, who denies the worshipable demigods, his own self and the Supreme Lord within himself, thus ruining all religious principles, and who is still infected by material contamination, is deviated and lost both in this life and the next. SB 11.18.40-41