Answer:
That a territory itself should decide if it wants to allow slavery within its borders
Explanation:
The Popular Sovereignty doctrine holds that all political power is inherent in the people, that is to say, people create and determine the nature of its government, gives consents to it and sustains it; people tend to accomplish this by electing their representative through fair elections, which should advocate for their interests.
Thus, this principle expresses that is the people itself who should decide if it wants to allow slavery, or any type of system, within its border.
