Scope of Sprint backlohg

Hi All,

I have gone through the scrum guide and other tests multiple times,but I couldn't get clear idea on below statements from scrum guide.

Development team collaborate with the product owner to negotiate the scope of sprint backlog within the sprint and development team can change sprint backlog through out the sprint??.

As per my understanding dev team can change the tasks/work in sprint backlog but guide says sprint backlog can be changed through out the sprint..can any one explain me this in detail.

Also what s the scope DEv team and PB negotiate?

Thank you