Abdullah (may Allah be pleased with him) said: When this verse was revealed the people felt it hard on them, so they said: “Who amongst us does not wrong himself?” So, the Prophet (saws) said:

“It does not mean this! Have you not heard what the pious servant (of Allah) said: {O my son! Join not others in worship with Allah. Verily joining others in worship with Allah is a great wrong indeed.”}. Verily, it refers to associating others in worship with Allah.”[1]

Therefore, what is meant is associating others in worship with Allah (swt). Association is not only worshipping idols or the moon, but it also includes believing that someone other than Allah (swt) can harm or give benefit, can give or reject, can join ties and cut them off, and can humiliate some people and raise the ranks of some others. The human being might even at times associate his desire with Allah (swt) as He says:

 أَرَأَيْتَ مَنِ اتَّخَذَ إِلَهَهُ هَوَاهُ

(Have you seen (O Prophet) the one who has taken their own desires as their god?) (Al-Furqan: 43)

(  أُولَٰئِكَ لَهُمُ الْأَمْنُ وَهُم مُّهْتَدُونَ… ) (…those will have security, and they are [rightly] guided): This is because, they do not mix their faith with wrongdoing, and thus, have pure genuine faith. They genuinely believe that Allah (swt) owns the proportions of life and death, and that no one can overcome His affairs and His Sovereignty.

[1] Musnad Imam Ahmad Bin Hanbal: Musnad al-Mukthireen min as-Sahabah, Musnad Abdullah Bin Mas’ud, Hadith No. 3589.