Dhau is a type of curd

Curd

Curds are a dairy product obtained by curdling milk with rennet or an edible acidic substance such as lemon juice or vinegar, and then draining off the liquid portion. The increased acidity causes the milk proteins to tangle into solid masses, or curds. The remaining liquid, which contains only...

 prepared by Newar

Newar

The Newa , Newār or Newāl) are the indigenous people and the creators of the historical civilization of Nepal's Kathmandu Valley. The valley and surrounding territory have been known from ancient times as Nepal Mandala, its limits ever changing through history.Newas have lived in the Kathmandu...

s. This is made by fermentation process of Milk in humid condition.

Dhau, curd, denotes purity and is associated with masculinity, whereas duru, milk, is associated with femininity, as well as with purity. Both are used as purifying substances in panchamrita and panchagabya, and their sacred properties are inherent from the cow.

Dhau sagan, curd, and baji are sent or given ritually on various occasions. It is regarded as a good omen; and giving it is a wish of purity for the receiver. Thus giving dhau sagan is to say "I wish You purity."