This statement of Allah, the Exalted, to His Messenger comforted the worries that the polytheists were causing him due to their statements directed towards him. This is just as Allah says about them,

[وَقَالُواْ مَا لِهَـذَا الرَّسُولِ يَأْكُلُ الطَّعَامَ وَيَمْشِى فِى الاٌّسْوَاقِ لَوْلا أُنزِلَ إِلَيْهِ مَلَكٌ فَيَكُونَ مَعَهُ نَذِيراً - أَوْ يُلْقَى إِلَيْهِ كَنْزٌ أَوْ تَكُونُ لَهُ جَنَّةٌ يَأْكُلُ مِنْهَا وَقَالَ الظَّـلِمُونَ إِن تَتَّبِعُونَ إِلاَّ رَجُلاً مَّسْحُوراً ]

(And they say: "Why does this Messenger eat food, and walk about in the markets. Why is not an angel sent down to him to be a warner with him'' Or; "(why) has not a treasure been granted to him, or why has he not a garden whereof he may eat'' And the wrongdoers say: "You follow none but a man bewitched.'') [25:7-8] Thus, Allah commanded His Messenger and guided him to not let these statements of theirs grieve his heart. Allah directed him to not let these statements prevent him, or deter him from calling them to Allah, both day and night. This is as Allah said,

[وَلَقَدْ نَعْلَمُ أَنَّكَ يَضِيقُ صَدْرُكَ بِمَا يَقُولُونَ ]

(Indeed, We know that your breast is straitened at what they say.) [15:97] Allah says in this verse,

[فَلَعَلَّكَ تَارِكٌ بَعْضَ مَا يُوحَى إِلَيْكَ وَضَآئِقٌ بِهِ صَدْرُكَ أَن يَقُولُواْ]

(So perchance you may give up a part of what is revealed unto you, and that your breast feels straitened for it because they say...) The meaning here is that he (the Prophet ) may be compelled to give up the Message due to what they (the polytheists) say about him. However, Allah goes on to explain: "You (Muhammad) are only a warner and you have an example in your brothers of the Messengers who came before you. For verily, the previous Messengers were rejected and harmed, yet they were patient until the help of Allah came to them.''