What are Pekudim?
Radak (8-9): Normally, they are [any] Mitzvos. Since later the verse mentions Mitzvas Hashem, here it must be Mitzvos of intellect, that Hashem Hifkid (deposited) and put in the heart; intellect teaches them. It is like "v'Hayah ha'Ochel l'Fikadon" (Bereishis 41:36), i.e. something stored.
Malbim: They are special Mitzvos via which we remember matters of Emunah, e.g. Mo'adim recall Yetzi'as Egypt, or Midos, e.g. Mitzvos to help unload, load, Pe'ah (leaving a corner of the field for the poor) and [other] Matanos Aniyim.
How are "Pekudei Hashem Yesharim"?
Radak: There is nothing crooked in them. If not, intellect would not teach them!
Malbim: Pekudei Hashem are based on straightness and Tzedek. They are unlike laws that people made based on their consensus; those laws are not straight.
Why does it say that Pekudei Hashem are "Mesamchei Lev"?
Radak: A Chacham rejoices over his Chachmah. When it overpowers the body and conducts it according to intellect, that is the greatest Simchah in the world; it is Simchah of the Nefesh. They are not called Mesamchei Adam, for man delights in worldly desires. However, the intellect (heart) delights only in ways of intellect - "Ki Sason Libi Hemah" (Tehilim 119:111).
Radak: (This is another way that Torah is better than the sun.) If a person sits in the sun too much, he worries about its heat. Hashem's Torah is different - it gladdens the heart.
Malbim: Laws that people made, do not give Simchah to the heart, for they do not have an honored purpose, which makes the heart rejoice when it knows the benefit that comes from the action.
Lev Eliyahu (Shemos p.57): Torah has all these attributes - Temimah, Meshivas Nafesh, Machkimas Pesi, Mesamchei Lev - when he dons Tefilin and subjugates his heart and Nefesh for Avodas Hashem.
What is "Mitzvas Hashem"?
Radak (8): It is what Hashem commanded to do to serve Him and love Him, like a master commands his slave.
Malbim: They are unlike people's commands; they go in darkness. The light of intellect did not shine on them.
How is Mitzvas Hashem "Barah"?
Rashi: It illuminates.
Radak: Every Mitzvah in its place is clear without dross.
Malbim: All Mitzvos Hashem have proper reasons to straighten mindsets and for happiness of the Nefesh.
Why does it say that Mitzvas Hashem is "Me'iras Einayim"?
Radak: A nation without Mitzvos goes in darkness, for Mitzvos illuminate the heart. They are a step towards the great honor.
Radak: (This is another way that Torah is better than the sun.) If a person looks at the sun, his eyes weaken. Hashem's Mitzvos are different; they enlighten the eyes.
Malbim: They illuminate the eyes, i.e. the senses. They teach mercy, grace, generosity and all good Midos in which a person will live in the world to come.
