This refers to another group of people who were oppressed in Makkah and whose position with their own people was weak, so they went along with them when they were tried by them. Then they managed to escape by emigrating, leaving their homeland, families and wealth behind, seeking the pleasure and forgiveness of Allah. They joined the believers and fought with them against the disbelievers, bearing hardship with patience. Allah tells them that after this, meaning after their giving in when put to the test, He will forgive them and show mercy to them when they are resurrected.
((Remember) the Day when every person will come pleading) meaning making a case in his own defence.
(for himself.) means, no one else will plead on his behalf; not his father, not his son, nor his brother, nor his wife.
(and every one will be paid in full for what he did,) meaning whatever he did, good or evil.
(and they will not be dealt with unjustly.) meaning there will be no decrease in the reward for good, and no increase in the punishment for evil. They will not be dealt with unjustly in the slightest way.