The queen excluder is a basic piece of equipment used by many backyard beekeepers. It is placed between the deep food chamber and the shallow (or medium) honey storage chamber, which is the part of the hive used to collect the remaining honey. As the name suggests, this gadget prevents or excludes the queen bee from entering the honey pot and laying eggs. Otherwise, the queen laying eggs in the nest will encourage the bees to bring pollen into the nest and may destroy the clarity of the honey. The grid is spaced in such a way that smaller worker bees can pass to the upper layers of the honey and fill them with honey only.
Many experienced beekeepers will not use queen excluders because it slows down the bees' honey production. Some say it can even lead to swarming. However, it takes a season or two of experience to determine when it is okay to not use the queen excluder (if you are sure you have a very strong colony, you may choose not to use the queen excluder). if you are sure there is a very strong nectar flow and that the bees are rapidly filling many cells with nectar). In the first year be safe and use a queen excluder. Next year Next year, you can try not using it when placing the upper layer of honey, timed to coincide with the main nectar flow.
Reference material:《Beekeeping For Dummies》