Allah, the Exalted, informs that they said to their Prophet,

This means that they claimed that Hud had not brought them any proof or evidence for what he claimed.

They were saying how could his mere statement, "Leave these gods," be sufficient proof for them to leave their idols?

This means that they did not believe what he was saying was true.

They were saying, "We think that some of our idols have afflicted you with madness and insanity in your intellect because you are trying to stop them from being worshipped and defame them."

{He said: "I call Allah to witness and bear you witness, that I am free from that which you ascribe as partners in worship besides Him (Allah)."} 11:54-55

Here, he is saying, "Verily, I am innocent of all of the rivals and idols (that you associate with Allah).

{So plot against me, all of you,} you and your gods if they are true."

{and give me no respite.} the blinking of an eye."

Then, Allah says,

Every creature is under His (Allah's) power and His authority. He is the Best Judge, the Most Just, Who does not do any injustice in His ruling. For verily, He is upon the straight path.

Verily, this argument contains a far-reaching proof and absolute evidence of the truthfulness of what Hud had come to them with. It also proves the falsehood of them worshipping idols that could not benefit nor harm them. Rather, these idols were inanimate objects that could not hear, see, befriend, or make enmity. The only One Who is worthy of having worship directed solely towards Him is Allah alone, without any partners. He is the One in Whose Hand is the sovereignty and He is in control of all things. There is nothing except that it is under His ownership, power and authority. Thus, there is no deity worthy of worship except Him and there is no Lord other than Him.