The thread count is usually used to describe the fabric density. In most case, it refers to cotton or poly-cotton (polyester blended with cotton as fibers) fabric. Many other man made fabrics are determined by other factors such as weight per square meters and so on.
The thread count is the number of threads vertical plus horizontal per square inch of fabric. Example 180+220 thread fabric will be described as 400-thread count.
The thread count cannot solely tell the fabric quality. In fact, very high thread count means fabric made from very thin thread thus the fabric will be less strong enough.
For a plain-weave fabric, 200-400 thread count should well for most of bed linen or sheets.