John is saying that Isaiah's prophecy is about himself. The word "voice" here refers to the person who is crying out in the wilderness. Alternate translation: "I am the one calling out in the wilderness" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Here the word "way" is used as a metaphor. Alternate translation: "Prepare yourselves for the Lord's arrival the same way that people prepare the road for an important person to use" (See: [[rc://en/ta/man/jit/figs-metaphor]])