The cloaca is the part of the bird’s body which stores urates, feces, urine and eggs. The vent or cloacal lips are used to control the passage and frequency of droppings etc. In a healthy budgerigar, the feathers around the vent will be dry and the vent itself clean.
A dirty, wet or clagged vent is not a good sign. Suggest you stop feeding anything other than seed and seek veterinary advice.