He said:

[يبُنَىَّ إِنَّهَآ إِن تَكُ مِثْقَالَ حَبَّةٍ مِّنْ خَرْدَلٍ]

(O my son! If it be (anything) equal to the weight of a grain of mustard seed, ) means, if a wrong action or a sin be equal to the size of a grain of mustard seed.

[يَأْتِ بِهَا اللَّهُ]

(Allah will bring it forth.) means, He will bring it forth on the Day of Resurrection, when it is placed in the Scales of justice and everyone is rewarded or punished for his actions -- if they are good, he will be rewarded and if they are bad he will be punished. This is like the Ayat:

[وَنَضَعُ الْمَوَزِينَ الْقِسْطَ لِيَوْمِ الْقِيَـمَةِ فَلاَ تُظْلَمُ نَفْسٌ شَيْئاً]

(And We shall set up Balances of justice on the Day of Resurrection, then none will be dealt with unjustly in anything) (21:47).

[فَمَن يَعْمَلْ مِثْقَالَ ذَرَّةٍ خَيْراً يَرَهُ - وَمَن يَعْـمَلْ مِثْقَالَ ذَرَّةٍ شَرّاً يَرَهُ ]

(So, whosoever does good equal to the weight of a speck of dust shall see it. And whosoever does evil equal to the weight of a speck of dust shall see it.) (99:7-8) Even if this tiny thing were to be hidden inside a solid rock or anywhere in the heavens and the earth, Allah will bring it forth, because nothing is hidden from Him, not even the weight of a speck of dust in the heavens or on the earth. Allah says:

[إِنَّ اللَّهَ لَطِيفٌ خَبِيرٌ]

(Verily, Allah is Subtle, Well-Aware.) meaning, His knowledge is subtle, for nothing is hidden from Him, no matter how small, subtle and minute.

[خَبِيرٌ]

(Well-Aware.) even of the footsteps of an ant in the darkest night. Then he (Luqman) said:

[يبُنَىَّ أَقِمِ الصَّلَوةَ]

(O my son! Perform the Salah,) meaning, offer the prayer properly at the appointed times.

[وَأْمُرْ بِالْمَعْرُوفِ وَانْهَ عَنِ الْمُنْكَرِ]

(enjoin the good, and forbid the evil,) meaning, to the best of your ability and strength.

[وَاصْبِرْ عَلَى مَآ أَصَابَكَ]

(and bear with patience whatever befalls you.) Luqman knew that whoever enjoins what is good and forbids what is evil, will inevitably encounter harm and annoyance from people, so he told him to be patient.

[إِنَّ ذَلِكَ مِنْ عَزْمِ الاٍّمُورِ]

(Verily, these are some of the important commandments.) means, being patient when people cause harm or annoyance is one of the most important commandments.

[وَلاَ تُصَعِّرْ خَدَّكَ لِلنَّاسِ]

(And turn not your face away from men with pride) means, `do not turn your face away from people when you speak to them or they speak to you, looking down on them in an arrogant fashion. Rather be gentle towards them and greet them with a cheerful face,' as it says in the Hadith:

«وَلَوْ أَنْ تَلْقَى أَخَاكَ وَوَجْهُكَ إِلَيْهِ مُنْبَسِطٌ، وَإِيَّاكَ وَإِسْبَالَ الْإِزَارِ فَإِنَّهَا مِنَ الْمَخِيلَةِ، وَالْمَخِيلَةُ لَا يُحِبُّهَا الله»

(... even if it is only by greeting your brother with a cheerful countenance. And beware of letting your lower garment trail below your ankles, for this is a kind of boasting, and Allah does not like boasting.)

[وَلاَ تَمْشِ فِى الاٌّرْضِ مَرَحًا]

(nor walk in insolence through the earth.) means, `do not be boastful, arrogant, proud and stubborn. Do not do that, for Allah will hate you.' So he said:

[إِنَّ اللَّهَ لاَ يُحِبُّ كُلَّ مُخْتَالٍ فَخُورٍ]

(Verily, Allah likes not any arrogant boaster.) meaning, one who shows off and admires himself, feeling that he is better than others. And Allah says:

[وَلاَ تَمْشِ فِى الاٌّرْضِ مَرَحًا إِنَّكَ لَن تَخْرِقَ الاٌّرْضَ وَلَن تَبْلُغَ الْجِبَالَ طُولاً ]

(And walk not on the earth with conceit and arrogance. Verily, you can neither rend nor penetrate the earth nor can you attain a stature like the mountains in height.) (17:37). We have already discussed this is detail in the appropriate place.