Screeding means to drag a long, flat board across a pliable material to smooth it down.
This also means that the board you use for this purpose is called a screed.
For example, you might have a wavy sub-floor that needs to be filled in with leveling compound. If the compound is not self-leveling, then you must drag the screed across the top to bring the compound to level.
A screed is a length of 2x4 or an aluminum bar that is pulled across wet concrete to smooth it down.
In the most basic applications, the 2x4 is set on edge and pulled backward (toward you) in order to generally smooth down the lumpy concrete. It helps to have two workers, one on each side. Aluminum screeds are often used, as they provide a straighter edge.
For more professional screeding, a motorized screed may be used that has a long handle to eliminate laborious bending and tugging.
Screeding is not the final finish. For that, you need a bull float.