Allah informs us of he who fears standing before his Lord, being frightened about matters between himself and Allah when he is not in the presence of other people. So, he refrains from disobedience and he performs acts of obedience when no one sees him except Allah.
Allah mentions that this person will have forgiveness and a great reward. This means that his sins will be remitted and he will be rewarded abundantly.
Then, Allah tells us that He is aware of the innermost conscience and secrets, meaning that which occurs in the hearts (ideas, thoughts etc)
Doesn’t the Creator know? And He is the Most Kind and Courteous, All-Aware (of everything).
Culled from Tafsir bn Kathir.