Sashiko is the Japanese art of visible mending. Visible, because you see it, and mending because it repairs and re-inforces the fabric.
Here's an article on the History of Sashiko and repair and decoration which offers a good overview.
We've found some tutorials to show you different styles. This tutorial from tikkido shows you the most basic, simple method of sashiko mending, using a simple running stitch. But just because the technique iseasy doesn't mean you can't make beautiful things. You can stick with classic white on blue or play with all sorts of colours.
Sashiko Mending - TUTORIAL by Tikkido
Sashiko Mending - TUTORIAL by Mrs. G's Sewing Space
Sashiko Mending Basic Procedure by SashiCo 刺し子
Visible Mending on Denim by WonderFil Threads