Lambswool is wool

Wool

Wool is the textile fiber obtained from sheep and certain other animals, including cashmere from goats, mohair from goats, qiviut from muskoxen, vicuña, alpaca, camel from animals in the camel family, and angora from rabbits....

 which is 50mm or shorter from the first shearing

Sheep shearing

Sheep shearing, shearing or clipping is the process by which the woollen fleece of a sheep is cut off. The person who removes the sheep's wool is called a shearer. Typically each adult sheep is shorn once each year...

 of a sheep, at around the age of seven months. It is soft, elastic, and slippery, and is used in high-grade textiles.

Weaner fleece is wool 50mm or longer from young sheep, that have been shorn for the first time, and which exhibits the characteristc lambs tip and staple structure.