As narrator, Esperanza describes the trees that she often looks as the only things around that understand her:
When I am too sad and too skinny to keep keeping, when I am a tiny thing against so many bricks, then it is I look at trees.... Four who grew despite concrete. Four who reach and do not forget to reach. Four whose only reason is to be and be.
Further, she notes how the trees send down roots to grab the earth and hold themselves stable. Thus, they set an example to Esperanza to establish a foundation for herself in which she will have roots, and to continue to aspire for her vision of the future in spite of her meager living conditions and environment. She does not remain in her house like Mamacita, who refuses to try to learn English, or like Rafaela, whose husband locks her in their apartment all day while he works. Esperanza, whose name means "hope," understands that the young, vibrant trees do not belong in this decaying barrio, either.
Like the trees, Esperanza understands that she must grow and reach beyond her meager and stultifying environment.
Esperanza identifies with the trees. The trees have “skinny necks and pointy elbows” like her; also like her, they are out of place (they “do not belong here but are here”). The trees inspire her; she hopes to be strong like the trees (their “secret strength” that comes from “never quit[ting] their anger”), to endure despite all hardships. The trees provide emotional support for each other; if one should “forget his reason for being,” the trees “would droop like tulips in a glass, each with their arms around the other”—an example of the support Esperanza would like to receive (and give). In this context, it’s interesting that her “understanding” of the tree is itself a kind of secret; she and her sister can both hear them from their room, but only Esperanza can hear their insistent message: “Keep keep keep.” The trees teach that, no matter what, Esperanza must continue “to be and be.”
Here's an excerpt from the enotes analysis on the vignette, go to the link below if you need more information:
The four skinny trees are a source of inspiration for Esperanza. More than any person or thing so far in the novel, the four trees give Esperanza strength and encouragement. This is because Esperanza sees herself in them, and them in her. The likeness is more than physical. Though they also have “skinny necks and pointy elbows,” the other similarities are more significant.
Like Esperanza, the trees do not belong on Mango Street. They belong somewhere else, somewhere better, somewhere with more room to grow; but, like Esperanza, they have been put on Mango Street against their will. Though they are skinny, they are strong—strong enough to grow even though they are surrounded by concrete instead of grass. Likewise, Esperanza is strong enough to grow in an environment in which she is restricted by her race, class, and gender.
The " Four skinny tress" symbolizes The four girls that were in the book. They used this because The four skinny trees are always changing and there is nothing they can do.