Screeding is the process pulling/scraping a screed board along a pair of parallel screed rails, the purpose being to level out the material between the two screed rails, be it crushed stone or a coarse bedding sand.
Man screeding crushed stone for paver patio

The trick to screeding is to keep the screed board tight to the screed rails without putting too much pressure on the rails.  That can bend them or push them deeper into the base material, making your base preparation come out uneven.