Hollis's comment, which can be found in early in chapter 1, is that she read somewhere that "if someone looks into your eyes," they will see into your soul. This is an uncomfortable thought for Hollis, because she doesn't want anyone seeing into her soul. For this reason, she has her A&S baseball hat pulled low to hide her eyes as much as possible. In a nutshell, what she means by seeing into another person's soul is getting to know someone well. Hollis has no interest in getting to know yet another foster family—or in having them get to know her.
It is immediately apparent that Hollis is a foster child who has been shunted from one home to the next. In this life, where everything and everybody seems temporary, it is natural for Hollis to not want anyone to see into her soul. She sees little point in becoming attached to anybody or to letting anybody really get to know her. With this being the fifth house that she has been sent to, the houses and the foster parents become less and less meaningful.
In a place where Hollis feels she is being dumped "like the UPS guy dumping off packages on a busy day," Hollis has no interest in letting anyone get close to her.