A Parable for the Weakness of the False Gods of the Polytheists
`Ali bin Abi Talib said that Allah's statement,
لَهُ دَعْوَةُ الْحَقِّ
(For Him is the Word of Truth.) is in reference to Tawhid, according to Ibn Jarir At-Tabari. Ibn `Abbas, Qatadah, and Malik who narrated it from Muhammad bin Al-Munkadir, said that,
لَهُ دَعْوَةُ الْحَقِّ
(For Him is the Word of Truth.) means, "La ilaha illallah." Allah said next,
وَالَّذِينَ يَدْعُونَ مِن دُونِهِ
(And those whom they invoke besides Him...), meaning, the example of those who worship others besides Allah,
كَبَـسِطِ كَفَّيْهِ إِلَى الْمَآءِ لِيَبْلُغَ فَاهُ
(like one who stretches forth his hand for water to reach his mouth,) `Ali bin Abi Talib commented, "Like he who stretches his hand on the edge of a deep well to reach the water, even though his hands do not reach it; so how can the water reach his mouth" Mujahid said about,
كَبَـسِطِ كَفَّيْهِ
(like one who stretches forth his hand) "Calling the water with his words and pointing at it, but it will never come to him this way." The meaning of this Ayah is that he who stretches his hand to water from far away, to either collect some or draw some from far away, will not benefit from the water which will not reach his mouth, where water should be consumed. Likewise, those idolators who call another deity besides Allah, will never benefit from these deities in this life or the Hereafter, hence Allah's statement,
وَمَا دُعَآءُ الْكَـفِرِينَ إِلاَّ فِى ضَلَـلٍ
(and the invocation of the disbelievers is nothing but misguidance.)
Tafsir Ibn Kathir (abridged, English).