Prior to release planning and periodically as the product backlog evolves and the team gains insight through experience the Scrum Team will estimate the size of items on the Product Backlog.   This is key information to help the Product Owner make prioritization decisions (some items may become less of a priority if the Product Owner learns that major effort will be required to deliver them).

Estimations are relative and are measured in “points” rather than real-world units of effort.  To the uninitiated this may seem unpredicatable but after the first few sprints Scrum teams can empirically derive a velocity that allows them to accurately predict delivery capability.