Weight is a way of indicating the thickness of a thread.

The most common weight system specifies the length of the thread in kilometres required to weigh 1 kilogram.

Regular cottons threads usually have a weight of 40.

The lower the number the thicker the thread.

For example:
Heavier threads can have a weight of 20.
Thinner threads can have a weight of 60.

shopSewing and embroidery threads

Below are some examples of threads.