Allah tells us about His close Friend Ibrahim, peace be upon him, and how He bestowed upon him guidance aforetime, i.e., from an early age He inspired him with truth and evidence against his people, as Allah says elsewhere:

{And that was Our proof ivhich We gave Ibrahim against his people} [6:83].

The point here is that Allah is telling us that He gave guidance to Ibrahim aforetime, i.e., He had already guided him at an early age.

{and We were Well-Acquainted with him.} means, and he was worthy of that. Then Allah says:

This is the guidance which he had been given during his youth: his denunciation of his people’s worship of idols instead of Allah. Ibrahim said:

{"What are these images, to which you are devoted?"} meaning, which you worship with such devotion.

means, they had no other evidence apart from the misguided actions of their forefathers. Ibrahim said:

meaning, Speaking to your fathers whose actions you cite as evidence would be the same as speaking to you. Both you and they are misguided and are not following any straight path.’ When he called their intelligence into question, and said that their fathers were misguided and belittled their gods.

They said: These words that you are saying, are you speaking in jest or are you telling the truth? For we have never heard such a thing before.’

meaning, your Lord, beside Whom there is no other god, is the One Who created the heavens and the earth and all that they contain; He 'is the One Who initiated their creation; He is the Creator of all things.

means, and I bear witness that there is no God other than Him and no Lord except Him.