Confuse about a point related to Sprint.

Hello,

I have a question about Sprint as below:

In Scum-Guide (Version July 2013, page 7) , there is description about Sprint:

"During the Sprint:

.
.

.Scope may be clarified and re-negotiated between the Product Owner and Development Team as more is learned."

Could you help to explain what does "Scope" mean here ? I am confusing because there was some where say that: The number of work is not allowed change during a Sprint.


Thank you.