This is a technical term, much popular in fabric industry. It means the number of threads , both horizontally and vertically, in a square inch sized cloth. The general idea is , the more the thread count, the fabric will be of higher and softer quality. The regular thread count is 200 , i.e. horizontally there are 100 threads as well as vertically. But when the thread count exceeds 400 , the price of fabric increased proportionally.