Femininity is a quality of acting in a typically womanly, girlish, or feminine way. Different cultures have different ideas of what femininity looks like.
If you're a gentle, sweet-natured person, your grandmother may praise your femininity. You might argue, however, that your brother is even sweeter and more soft-spoken than you are. Society has a particular idea and definition of exactly what femininity is, but this varies widely for individual women. Femininity comes from the adjective feminine, "relating to women," from the Latin femina, "woman."