The Ladder of Eros or Desire:

First, see Platonic Idealism.

Because the soul yearns to return to its origin -- the Form Of The Good, the Form of Eternal Beauty etc. -- we yearn for love and beauty; this is what draws us to beautiful objects and explains desires, such sexual attraction or our love for good, beautiful bodies etc..  But as we become wiser, we begin to yearn for increasingly ideal and universal forms of love and beauty: we shift from physical attraction to loving beautiful souls (we "fall in love", which Plato equates with loving another's soul) -- then we realize eternal love extends to all people, so we desire a beautiful city where all souls are treated well -- as we grow wiser we desire increasingly "ideal" forms of goodness and thus turn to loving art and philosophy ("the love of knowledge") -- then we yearn for divine love and beauty (The Form Of The Good) that transcends life in the sensible world altogether.  This final yearning is the philosopher's staring directly at the sun (representing The Form Of The Good) in "The Allegory Of The Cave": the love of God.

The Ladder: each represents a step "up" from the previous, from the love of physical (sensible knowledge) toward the Ideal (intelligible knowledge):  Sexual Desire (Procreation) --> Care of Children --> Creativity (Art etc.) --> Love of "temperance and justice" (care of community, family, state) -- > Love of Knowledge (philosophy) --> Desire for the Ideal itself (for Good, Truth, Beauty (for god or God)

Two important things about Plato's "Ladder" concept:
a) It breaks the Greek tradition of Humanism so prevalent in Homer

b) It also aligns Greek philosophy with Hebrew theology and together these two will join to greatly influence Medieval Christian concepts of sexuality: lust is a debased form of love for something greater.  In general, Plato's philosophy urges us to turn away from the illusory physical, whether in the study of nature or the love of human bodies.