Calrifiation on Sprint back log

Hi All,

I have a few doubts on sprint backlog from the guide.

1)Scope can be re negotiated with the PO during the sprint?May I know what it exactly means
2)Dev team can change sprint backlog through out the sprint?Sprint backlog is not only tasks/work ,guide states its PBI's and plan to deliver .so only work can be changed as per my understanding,but guide says entire sprint backlog can be changed during the sprint.Can some one clarify on this?

Thank you
shalini