What are "Pekudim"?
Radak (1): They are Mitzvos that intellect teaches them; they are like a Pikadon (deposit) and something hidden in man's heart.
Malbim: They are Mitzvos that have testimony about Hashem's deeds and Hashgachah, e.g. Tefilin, Shabbos and Mo'adim. In them are Mufkad (stored) they matters that they teach about. Therefore, they are mentioned after Edus (refer to 119:2:1:3); via them, one remembers the Edus.
Why did Hashem command very much?
Radak: This is lest one err and sin. Rather, his entire heart [should be on the Mitzvos].
Ma'alos ha'Torah (DH v'Chen Matzinu): You commanded to guard Your commands, no matter what will happen to me.
Malbim: One must guard them like matters in which pearls and valuables are wrapped.
