Basically, we follow Janie for her whole life. Early on, we discover exactly what motivates her—the quest for true love—and for the rest of the novel, we follow her exploits in her pursuit of this goal.
Janie’s first two failed marriages rob her of her innocence, but this is an essential step toward achieving womanhood and maturity. Only by painfully discovering what love is not can Janie finally recognize true love for what it is in her relationship with Tea Cake. Because of this, adulthood is defined by suffering, learning lessons from that suffering, and using that knowledge to move forward toward one’s goal.
By the time she returns to Eatonville, Janie has absorbed a good deal of wisdom from her experiences...wisdom that she's eager to impart to Pheoby.