Thread count is a measurement of the fineness or coarseness of bedsheets. Thread count includes the total weft threads (horizontal threads) and warp threads (vertical threads) within a square inch of fabric. If the fabric is multi-ply (two or more layers of fabric stitched together), the threads in each extra later also count.
For example, a single-ply sheet set with 150 weft threads and 150 warp threads per square inch would have a thread count of 300. If that same sheet set was made from two-ply yarn, the thread count would be doubled to 600. A high thread count is not always an indicator of quality, and it only matters up to a point. A set of high-quality Egyptian cotton sheets might only have a 300 or 400 thread count and still feel like the best sheets you've ever used.