As any weaver who's worked with wool knows, fulling is a vital part of the process. Fulling gives the wool fabric more stability by enabling the fibers to expand and interlock, creating the robust woolens we know and love. In Scotland, the process of fulling is known as waulking. Traditionally, songs were sung as the cloth was beaten or stomped. Waulking songs were sung to fit the tempo of waulking, and so often you'd start with a slower song and then move to ones with faster tempos as the cloth got softer.
For most songs, one person would sing each verse and all the waulkers would take part in singing the chorus which would usually be nothing but nonsense words like "tra-la-la." While there are many traditional waulking songs, it would not be uncommon for the lead singer to mix and match verses, leave out parts, or even improvise their own lyrics. Waulking was usually done by the women of a village, and the songs would often have themes relating to their lives, many focusing on themes surrounding love.
While traditional waulking is rare these days, there are groups working to preserve the songs and the waulking that accompanied them. Sgioba Luaidh Inbhirchluaidh is one such group. You can see them in action in this video as they sing and work the wool.