Praise be to Allaah.
Ibn Katheer said: i.e., the people will be divided into three groups on the Day of Resurrection.
Al-Tabari said: thus they will be divided into the three groups mentioned at the end of the soorah at the time when they are dying.
Then Allaah explained the state of the (three) groups. He said “those on the Right Hand” to point out how great is their status and how respected they are.
“And those on the Left Hand (i.e. those who will be given their Record in their left hands) how (unfortunate) will be those on the Left Hand”; their being on the left indicates how terrible their situation will be.
This means that those who were foremost in doing good deeds in this world will be foremost in the Hereafter and will be the first to enter Paradise. Those who are of this caliber are the ones who will be closest to Allaah in gardens of delight, in the heights of ‘Illiyeen, in a lofty status above which there are no higher status. We ask Allaah to make us among those who are foremost. And Allaah knows best.
See Tafseer Ibn Sa’di, p. 832