The poem is about a mother's fear that her daughter will be "turned into" something other than her daughter. This fear is can be understood in several ways.

In one sense, the poem is about the mother's fear of what others ("them") will do to her daughter. In the first stanza, she is afraid her daughter will turn into a swift, a fast-flying bird, and that her freedom will take her away. In the second and third stanzas, she is afraid she'll be turned first into a princess, then a queen—both forms of royalty creating a barrier between mother and daughter. The sense that these are things that will happen to her daughter suggests that neither one has much agency over their fate.

In another sense, the poem is about growing up and the fear parents face over losing their children to time. In this case, the threat to her daughter comes not from any specific person or people, but the larger and more frightening world of experience. There is a sense that even though the daughter could become many things and leave her mother, the mother herself is unchangeable.

These are basic fears any parent knows. The poem's use of repetition and metaphor resembles a nursery rhyme or fairy tale, which gives the poem a childlike quality in keeping with the mother's desire that her little girl stay the way she is.

The central idea of the poem "Fear" is the very real fear the mother has of losing her daughter. She is afraid of people coming into her daughter's life and trying to give the daughter any ideas about leaving her mother. The mother is so afraid that her daughter will grow up and not want to be around her anymore. She is afraid that she won't be able to protect her daughter if she goes off on her own. The overwhelming fear that she feels rules her life. There is nothing in the poem that would give a reason for the mother's overwhelming fear. We are left on our own to determine what might have caused it. Maybe the mother was alone and only had her daughter, and the fear that the daughter would leave her alone made her feel the way she did. We just don't know, but we can feel the mother's fear.

"I don't want them to make
my little girl a princess.
In tiny golden slippers
how could she play on the meadow?
And when night came, no longer
would she sleep at my side.
I don't want them to make
my little girl a princess."

Although the fear seems to be unfounded, anyone who is a parent can relate to some degree to the poem. We all want to hold onto childhood a little longer. Childhood goes by so fast, and we want to hold our children close, but we also know that we have to let go at some point. 

Central to the poem "Fear" by Gabriela Mistral is a mother's anxiety about losing her child.  This is, in part, a selfish apprehension as the mother worries that the child will become like a swallow and metaphorically "fly off" to be with others--teachers, classmates, friends, and not her.  That the mother is a poor person rooted to one place is evidenced in this metaphor that depicts the child like a bird escaping her sight.  Also the mother worries that the child will leave her little "straw bed" and become "a princess." (lines 9-10), for if she becomes a princess, then the metaphorical "they" may make her a queen; with their royal obligations, the princess and queen will not be able to be together.  Here, then, is also the expression of fear for the daughter as life's obligations and pitfalls meet her.

Through the use of metaphor and repetition, the mother expresses her fear of her daughter's growing up and leaving her and encountering potential harm. She combs the girl's hair and does other physical things that she may keep the child close as long as she can.