Overall meaning : Allah Almighty strikes a parable to depict the condition of those who worship false gods besides Him and warns those who commit such deadly sin of the severest type of punishment:
Those who take protectors (false deities) besides or instead of than Allah, thereby seeking their benefit and victory, may be compared to a spider that builds itself a house, which is clearly the flimsiest of all houses. Likewise, these protectors will not be able to benefit them in any way. Were the idolaters to know that this is their example, they would not worship idols besides or instead of Allah!
Allah knows whatever deities they invoke besides or instead of Him, and He knows their condition and their weakness, and He will punish their worshippers for their unbelief. Allah is the All-Powerful, the All-Wise in His Law.
These parables which we set forth in the Qur’an in order to open people’s eyes and hearts to the truth can only be understood by those who are endowed of knowledge of Allah and His revelations and laws.
The Almighty then one of the signs of His ultimate power: Allah has created the heavens and the earth for a definite, wise purpose, and not in vain. Surely in this there is a sign of Allah’s oneness and being the sole creator and disposer of all affairs for true believers.