Allah chastises and admonishes those who stayed away from Jihad and refrained from performing it, even though they had the supplies, means and ability to join it. They asked the Messenger for permission to stay behind, saying,
{"Leave us (behind), we would be with those who sit (at home)"}

thus accepting for themselves the shame of lagging behind with women, after the army had left. If war starts, such people are the most cowardice, but when it is safe, they are the most boastful among men. Allah described them in another Ayah,
{Then when fear conies, you will see them looking to you, their eyes revolving like (those of) one over whom hovers death; but when the fear departs, they will smite you with sharp tongues.} [33:19]
their tongues direct their harsh words against you, when it is safe to do so. In battle, however, they are the most cowardice among men.
Allah said in another Ayah,
{Those who believe say: "Why is not a Surah sent down (for us)? But when a decisive Surah (explaining and ordering things) is sent down, and fighting is mentioned therein, you will see those in whose hearts is a disease looking at you with a look of one fainting to death. But it was better for them. Obedience (to Allah) and good words (were better for them). And when the matter is resolved on, then if they had been true to Allah, it would have been better for them.} [47:20-21]
Allah said next,
{Their hearts are sealed up} because of their staying away from Jihad and from accompanying the Messenger in Allah's cause,
{so they understand not.} they neither understand what benefits them so that they perform it nor what hurts them so that they avoid it.