peace,
if i read this ayat/verse, without the brackets, i understand: the prophets taking the covenant of Allah, are asking the followers: "i have given u the Book and the wisdom from Him."
"later, a messenger as conformer of the wisdom u have, will come." "u - the followers, firmly believe and assist the messenger." "So, will u adhere and agree this", said the prophet. the followers answered "yes, we'll adhere". then the prophet said, "bear witness, and i'll also be a witness among witnesses".
i understand that every prophet, say these words, to their followers. as, the messengers will be coming till the judgement day to prove the Ayats of Koran.
note: i'm new to Koran. i read in my native language. i come from an idol worshiping family. i left idol worshiping after reading Koran. Koran gives me so much meaning to my life.