Reason for Revelation: This verse was revealed to encourage the battle of Tabuk. Upon returning from Taif, Prophet Muhammad (saws) announced preparations for Jihad against the Romans. This occurred during a difficult time for the people as the heat was extreme, fruits were ripe, and resting in shadows were pleasant. The Prophet Muhammad (saws) used to conceal the destination of the battles and mentioned a different one. However, in the case of the Tabuk battle, since it was to be undertaken during scorching heat, covering a long distance, and facing a formidable enemy, therefore he clearly told the Muslims of their destination so that they could prepare to meet their enemy. Thus, going out for the battle was very difficult for the companions, and they were sluggish. So that, Allah (swt) revealed this verse.
(انفِرُوا) (you are told to go forth): The Arabic word means to go out to purpose.
(اثَّاقَلْتُمْ إِلَى الْأَرْضِ ۚ) (you adhere heavily to the earth?): It is as if a heavy burden was placed upon you, and you were pressed down onto the ground.
(أَرَضِيتُم بِالْحَيَاةِ الدُّنْيَا مِنَ الْآخِرَةِ ۚ) (Are you satisfied with the life of this world rather than the Hereafter?): Are you contented with the pleasures of this worldly life? What does this worldly life offer in comparison to the Hereafter, especially for the believers, who fully know that the enjoyments of this worldly life are limited, compared to the Hereafter? It is a great loss to be content with the worldly life over the Hereafter. The worldly life is a transient trip, a changeable world where there is no stability. The young grows, the healthy falls ill; and the wealthy becomes poor. Such is the nature of life. Whatever you acquire of possessions and delights in this worldly life, no matter how grand and significant, is but meager in comparison to the Hereafter.