Sashiko thread is made of cotton, is relatively thick, and has a matte finish. Unlike embroidery floss, it is spun in such a way that the threads cannot be separated; it is used as is.
These are 40 m skeins of thread; the price is for 1 skein. When you remove the label, there is a large circle of threads bundled together at one point. Cut at the knot, and then you can twist or braid the threads and pull them out one by one from the top. This prevents knots, and the length is suitable for sewing.
Sashiko thread is a bit "fluffier" than embroidery floss, and it adheres nicely to the fabric you are sewing into, blending in over time as an integrated part of the material.
To learn a little more, you can watch this video, which is an introduction to the technique and equipment.
Search for sashiko on YouTube, and you'll find even more! There are also dedicated sashiko groups on Facebook, for example, and several craft associations offer sashiko courses, so check a craft association near you if you're interested! The book Mending Matters also covers a lot of sewing inspired by Japanese handicraft techniques, and The Ultimate Sashiko Sourcebook goes into more detail on how to draw and sew many different patterns, etc.
You can find needles and thimbles for sashiko here :)