Yarn count expresses the coarseness or fineness of yarn and is measured using various systems. There are indirect and direct count systems, with indirect systems like cotton using length per unit weight and direct systems like jute using weight per unit length. Various instruments can measure count, including the quadrant balance for short lengths, warp reel and balance method for longer lengths, and Beesley's balance for small fabric samples. Count affects properties like thickness and strength and is an important consideration in textile manufacturing.