The Mishnah continues its description of the order of the Pesach night. After the platter with all its items has been brought before him, a second cup of wine is poured for him. According to one understanding, and this is indeed the practice, the platter mentioned in the previous Mishnah was removed from the table and then returned to its place - so that the children will notice the change and ask.
"V'chan haben sho'el" - the questions of Mah Nishtanah:
At this point the son asks his father the questions of "Mah Nishtanah," and if the child does not know how to ask - his father teaches him. These are the four questions: "Mah nishtanah halailah hazeh mikol haleilos?" - How is this night different from all other nights?
Matzah: On all other nights we eat chametz or matzah, and on this night - matzah only.
Maror: On all other nights we eat other vegetables, and on this night - maror. We do not say "only maror," because at the beginning of the Seder a small amount of vegetable is eaten for the karpas; rather, the main vegetable that is eaten is the maror.
Roasted meat: This question does not appear in the text we have, but it was in use in the days of the Beis HaMikdash - on all other nights we eat meat that is roasted, thoroughly cooked, or cooked in the usual manner, and on this night specifically roasted meat. The reason is that the Pesach offering may be eaten only roasted and not cooked.
Dipping: On all other nights we dip once, and according to another version - we do not dip even once. This is the way the Gemara adjusts the reading of the Mishnah: that there is no particular mitzvah to dip even once. And on this night we dip twice - the karpas in salt water, and the maror in charoses.
The question that was replaced - the question of leaning:
It should be noted that in the text we have there is an additional question, which the Rambam includes: on all other nights we eat either sitting upright or leaning, and on this night we all lean. Some understand that this question was inserted into the text at a later stage, once the question about the roasted meat was no longer applicable. In the time of the Beis HaMikdash people regularly leaned at every formal meal, and therefore leaning was not a change at all; but nowadays, when it is not our way to lean, leaning has become something unusual, and from this the question was born.
"Maschil bignus umesayem b'shevach":
The Mishnah continues and establishes that the father teaches his son according to the child's level of understanding. When we tell the story of our people, we begin with the disgrace - with the fact that our forefathers were originally idol worshippers - and we go on to describe how the Holy One, Blessed is He, drew us close to His service and ultimately redeemed us.
The various expositions that we read in the Haggadah are derived from the declaration in the passage of Mikra Bikkurim - the declaration that a person is required to recite when he brings his bikkurim, his first fruits, to the Beis HaMikdash. We take those very verses, which describe the history of the Jewish people, and expound various points from them, until we complete that entire passage and conclude the expositions connected to it.