Istighfār comes from the Arabic root غ ف ر, associated with forgiveness and covering faults. Saying أَسْتَغْفِرُ اللَّهَ means “I ask Allah to forgive me.” Reciting the words should go together with sincere efforts to improve conduct.
If someone has taken another person's right, seeking forgiveness does not remove the responsibility to restore that right and correct the wrong.
In صَحِيفَتِهِ, the ending هِ means “his,” referring to the person's own record of deeds.