حَرِّضِ(Encourage): To urge and intensify the desire for something. Indeed, Allah (swt) commanded His Prophet (saws) to encourage the believers and boost their enthusiasm on the day of the Battle of Badr to fight against the polytheists, since they were to choose between seizing the caravan and the fight with their small number.

إِن يَكُن مِّنكُمْ عِشْرُونَ صَابِرُونَ يَغْلِبُوا مِائَتَيْنِ (If there are among you twenty steadfast, they will overcome two hundred,): One against ten, this is an extremely high standard of faith. The battle is a matter of faith; the power of faith in Allah (swt) and His Prophet (saws) makes the strength of a believer equals to the strength of ten disbelievers.

(وَإِن يَكُن مِّنكُم مِّائَةٌ يَغْلِبُوا أَلْفًا مِّنَ الَّذِينَ كَفَرُوا) (And if there are among you one hundred [who are] steadfast, they will overcome a thousand of those who have disbelieved.): Would it not be more concise to say ‘One believer overcome ten disbelievers? However, we have to take into consideration the various techniques in the battles and the skirmishes in regard of the number of fighters. There may be a unit of ten fighters, or a group of twenty ones. Here, Allah (swt) clarifies these groups that were fighting in the Battle of Badr, specifically.

(بِأَنَّهُمْ قَوْمٌ لَّا يَفْقَهُونَ) (because they are a people who do not understand.): Meaning, they neither understand nor know, and they do not believe that this worldly life is nothing but as a farm for the Hereafter, where there will be both, a paradise and a hell. Therefore, the believer has patience and perseverance greater than those disbelievers have.