Megillah, Chapter 4, Mishnah 5. The Mishnah discusses the person who is called up to read the Haftarah from the Prophets, and the additional honors that are given to him: "HaMaftir baNavi - hu poreis al Shema, v'hu over lifnei hateivah, v'hu nosei es kapav" - the one who reads the Haftarah from the Prophets is the one who leads the congregation in the berachos of Shema, he is the one who passes before the ark to lead the prayers, and he is the one who lifts his hands for the priestly blessing. These three privileges are given to him as compensation for the lack of honor involved in being called up to read from the Prophets.
Why is compensation needed?
Reading from the Prophets, as opposed to receiving an aliyah to the Torah, was considered in the time of the Mishnah to be a lesser honor, and in order to compensate for that, he was given other roles in the prayer service.
The three honors:
"Poreis al Shema" - as we learned in the earlier Mishnayos and in Mishnah 3, according to the simple understanding this is the person who fulfills the obligation on behalf of the congregation, having them in mind in the berachos of Shema, and this honor is given to him.
"Over lifnei hateivah" - he is the one who recites the repetition of the Shemoneh Esrei and fulfills the congregation's obligation of Kedushah.
"Nosei es kapav" - if he is a kohen, he is the one who goes up to the platform to bless the people.
At first glance this is difficult: after all, every kohen goes up to the platform, so in what way is the one who reads the Haftarah different from anyone else? Two approaches are offered in explanation:
It is indeed something that all the kohanim do, and the Mishnah mentions it here only as a contrast to the following Mishnah, which explains that a minor is not the one who lifts his hands for the priestly blessing.
The one who reads the Haftarah will be first, opening the priestly blessing and leading the berachos that the kohanim recite.
"V'im hayah katan - aviv o rabbo over al yado" - and if he was a minor, his father or his teacher passes before the ark in his place:
If the one who received the Haftarah was a minor who had not yet reached the age of mitzvos - which is permitted, and so the halachah is in practice - he cannot fulfill the congregation's obligation in the berachos of Shema, nor can he recite the repetition of the Shemoneh Esrei and Kedushah. So too we will learn below that he does not go up to the platform even if he is the only kohen. Therefore his father or his teacher passes before the ark in his place: they represent him and take these honors instead of him.