Fearing should be from Allah (swt) only, and the true believer does not fear any human because he is certain that all power belongs to Allah (swt). So, he fears none but Allah (swt) when he embarks on anything. Therefore, out of fearing Allah (swt) is to commit to His (swt) orders. Fearing is a feeling towards a one you think he can harm you, whereas in fact, none except Allah (swt) is able of bring benefits or cause harm. 

(… بِالْغَيْبِ … ) (…unseen …): They fear Allah (swt) despite that they never have seen Him with their eyes, rather they recognize Him (swt) through His creations. (unseen) may refer as well to unseen matters that they do not see but Allah (swt) informs them about. So, these unseen matters became clear to them as what they really see with their eyes. Another interpretation of this verse is that the believers fear Allah (swt) even when they are completely alone, where no one sees them, unlike those who pretend this attitude in from of people. Accordingly, these believers:

(  لَهُم مَّغْفِرَةٌ وَأَجْرٌ كَبِيرٌ… ) (…will have forgiveness and great reward): Thereof, fearing Allah (swt) when associated with recalling His punishments and longing to His reward make a person as humble, fasting, charitable, truthful and patient Muslim, as Allah (swt) says:

 إِنَّ ٱلۡمُسۡلِمِينَ وَٱلۡمُسۡلِمَٰتِ وَٱلۡمُؤۡمِنِينَ وَٱلۡمُؤۡمِنَٰتِ وَٱلۡقَٰنِتِينَ وَٱلۡقَٰنِتَٰتِ وَٱلصَّٰدِقِينَ وَٱلصَّٰدِقَٰتِ وَٱلصَّٰبِرِينَ وَٱلصَّٰبِرَٰتِ وَٱلۡخَٰشِعِينَ وَٱلۡخَٰشِعَٰتِ وَٱلۡمُتَصَدِّقِينَ وَٱلۡمُتَصَدِّقَٰتِ وَٱلصَّٰٓئِمِينَ وَٱلصَّٰٓئِمَٰتِ وَٱلۡحَٰفِظِينَ فُرُوجَهُمۡ وَٱلۡحَٰفِظَٰتِ وَٱلذَّٰكِرِينَ ٱللَّهَ كَثِيرٗا وَٱلذَّٰكِرَٰتِ أَعَدَّ ٱللَّهُ لَهُم مَّغۡفِرَةٗ وَأَجۡرًا عَظِيمٗا

(Indeed, the Muslim men and Muslim women, the believing men and believing women, the obedient men and obedient women, the truthful men and truthful women, the patient men and patient women, the humble men and humble women, the charitable men and charitable women, the fasting men and fasting women, the men who guard their private parts and the women who do so, and the men who remember Allāh often and the women who do so – for them Allāh has prepared forgiveness and a great reward.) (Al-Ahzab: 35)

Such Muslims who have these characteristics that are mentioned in the above verse, Allah (swt) grants them forgiveness and purification from all the sins, before being given a great reward. A believer gets his reward for every deed is a specific time. In other words, a reward which a person gets for a worldly deed comes to an end in this life, because the man himself ends with death, whereas the reward of the hereafter lasts forever, the believer will not miss it, neither the reward will miss the believer. This is the great reward. Allah (swt) says:

 وَمَا تُقَدِّمُواْ لِأَنفُسِكُم مِّنۡ خَيۡرٖ تَجِدُوهُ عِندَ ٱللَّهِ هُوَ خَيۡرٗا وَأَعۡظَمَ أَجۡرٗاۚ

(And whatever good you put forward for yourselves – you will find it with Allāh. It is better and greater in reward.) (Al-Muzzammil: 20)

We seek the reward for our righteous deeds from The One who does not deny it and saves it for us