What does this mean in the tech specs for a range hood "Duct Size 3-1/4" x 10", 6" Round." What does thatme?

we are building our own home andneed to cut out the roof opening for all the vents. The tech specs for a range hood blower are onsidering list this data which is confusing: "Duct Size 3-1/4" x 10", 6" Round." Can anyone explain what this means?