Kṛṣṇa clearly states in the Gītā that all actions in this world produce a reaction. Actions are defined as karma (pious actions according to Vedic injunctions), vikarma (impious actions contrary to Vedic injunctions) and akarma (those activities that do not produce any material result). Engaging in karma and vikarma forces the living being to remain in this material world, reincarnating in various species of life. Akarma refers to those actions that are dedicated to Kṛṣṇa – such devotional activities transcend all material reactions and bring one closer to the Supreme. You can read more about karma in Bhagavad-gītā Chapter 3 – Karma Yoga (The Yoga of Action) and Chapter 4 – Jñāna Yoga (The Yoga of Knowledge)