A new Queen
When the colony decides there are too many bees in the hive, they will make a queen cell.
This usually happens during the height of their season between May to June (but can happen a month or two later but these are much smaller in size), especially if they have all been cramped up together and unable to get out due to poor weather.
They will make a new Queen cell in order for her to take over the laying, and with a portion of bees the old queen will swarm and leave the hive to find a new home. A new queen will hatch and the remaining bees carry on accepting her.
This is a queen cell, noticeably different to rood cells