Tuk is a Gurmukhi term for a line of Gurbani. The Sikh holy Scripture is called the Sri Guru Granth Sahib. This is the current Guru or Guide for the Sikhs. Each full line of text in this scripture is referred as a Tuk.
Several Tuks make up a Shabad or Salok, etc. And together these Shabads form a Gurbani Raga or Chapter and there are 31 Ragas in the SGGS. Together the 31 Ragas form the 1430 pages of this sacred Gurbani called the Sri Guru Granth Sahib.