A Mitzva
A Mitzva is usually thought of as a set of bodily actions or rituals that we must carry out. But those actions are, in fact, only symbols of inner actions one should do.
The Book of Zohar (vol. 1 p. 242 in the Sulam commentary) explains that our soul consists of 613 desires. In the beginning, the soul is “corrupted.” This means that our 613 desires are oriented toward self-gratification,
or “egoism.”
We “correct” our soul by learning how to use each desire to benefit others. This is called “altruism.” A correction of a desire is called “keeping a Mitzva.” Since our soul consists of 613 desires, it is written that one must keep 613 Mitzvot ((plural for Mitzva.)
“Each item in the 613 Mitzvot in the Torah revolves on the axis of the one Mitzva, ‘love thy friend as thyself.’ This axis can only be sustainable within the framework of a whole nation, whose members are all willing and ready for it.”
Rabbi Yehuda Ashlag (Baal HaSulam), "The Revelation of Godliness"