Allah the Exalted here explains His order to Muslims to march forth with the Messenger of Allah ﷺ for the battle of Tabuk.

We should first mention that a group of the Salaf said that marching along with the Messenger ﷺ, when he went to battle, was at first obliged on all Muslims, because, as they say, Allah said,

انْفِرُواْ خِفَافًا وَثِقَالاً

Commentary

It will be realized that the expedition of Tabuk has continued appearing as an important subject in Surah At-Taubah. For this battle, a general call of Jihad was given asking all Muslims to participate in it. It was not permissible to contravene the order without a valid excuse. Those who did that were mostly hypocrites who have been mentioned in several verses appearing earlier. Then, some sincere Muslims had also remained behind due to their incidental laziness. Their repentance was accepted by Allah Ta` ala. Obviously, from all these events it can be gathered that the going forth of all Muslims in every Jihad is fard (obligatory) and staying behind is Haram (unlawful) -although, this is not the religious law. In fact, Jihad during normal conditions is Fard al-Kifayah, a collective obligation. It means that, should a number of Muslims sufficient for Jihad keeps engaged in Jihad, rest of the Muslims stand absolved of the obligation. However, if the number participating in Jihad turns out to be insufficient to the limits of being overpowered, it becomes obligatory on Muslims living nearby that they should come out to strengthen them and fight with them in the Jihad. If they too are not sufficient, the obligation falls on those near the later, and if they too are not sufficient, it falls on Muslims adjoining the later. This could reach the limit when, under such emergent conditions, Jihad becomes Fard ` Ain (absolute individual obligation) on Muslims of the whole world, and it will be Haram (unlawful) to avoid participating in jihad. Similarly, it could also become equally obligatory (fard) if the Amir of Muslims, seeing the need, gives a general call and invites Muslims to join the mission of Jihad. At that time too, participation in Jihad becomes obligatory (fard) and staying behind becomes haram (unlawful) - as it happened in the expedition of Tabuk due to the general call of Jihad given for it. The verse cited above clarifies that this order to join the Jihad of Tabuk was a specific order due to the general call for it. Under normal conditions, Jihad is not an individual obligation (Fard al-'Ain فَرض العین) that would require all Muslims to go forth in Jihad as a matter of obligation - because, like Jihad, there are other collective concerns and important undertakings of Islam and Muslims that are Fard al-Kifayah, also like Jihad. To fulfill these responsibilities too, different groups of Muslims have to follow the principle of division of work. Therefore, all Muslims should not go forth in every Jihad. The submission here may also help you understand the reality of Fard al-Kifayah. The, areas of work that are not individual, but collective in nature, and the responsibility of carrying these out falls on all Muslims, are areas that the Shari’ ah of Islam has declared to be Fard al-Kifayah. The purpose is that everything keeps being taken care of in its respective area and that all collective duties continue to be fulfilled. Duties such as washing, shrouding, offering Salah of Janazah and burying deceased Muslims, the making and maintaining of mosques, Jihad and the guarding of Islamic frontiers are all separate duties that are subject to the same rule of Fard al-Kifayah. In other words, the responsibility of fulfilling these obligations though falls on the Muslims of the whole world as a matter of principle but, should some of them were to do that satisfactorily enough, other Muslims also stand absolved of the obligation. An important area of work that falls within the imperatives of this Fard al-Kifayah is the religious education. That it is an obligation has been particularly mentioned in this verse by saying that this obligation should not be left out unattended even during the period of engagement in as important a duty as Jihad. The way in which it could be accomplished is that a smaller section from out of every large section sets forth for Jihad while the rest of people keep engaged in the pursuit of religious education. Then, those so educated should orient Muslims going on Jihad as well as teach and train others in their religion.