In Katherine Paterson's Lyddie, the protagonist is not technically a slave, but her position as an indentured servant paying off a debt is not much better in practical terms. Diana is fighting for the labor rights of the women at the textile mill and has clearly thought more clearly and conceptually than Lyddie has about the nature of freedom and slavery.
When she tells Lyddie that “the nature of slavery to make the slave fear freedom," Diana is expressing an important psychological truth, which can also be applied to prison, or even to military life. People crave certainty. While the life of a slave is harsh and disagreeable, it does at least provide all the certainty and structure anyone could want. The slave's will is subordinated to the will of another, and she is told exactly what to do and when to do it.
After a fairly short time, obedience becomes habitual, and the slave fears freedom. This is because freedom does in fact contain an element of risk. If you are free, you are responsible for your own destiny, and you make your own decisions to achieve a result that you decide is valuable. This situation contains an element of risk which is not present if you are merely doing what you are told to do.