Finishing stitches are used to securely end a piece of thread by anchoring it to the fabric.
Also see Holding stitch for how to use this method to finish your thread.
Working in an area that will subsequently be hidden beneath stitching, make two tiny stitches. Alternatively (if this is the last stitching in this area), carefully angle your needle and bring it up between previous stitches and make two tiny stitches, hiding them under your existing stitches.
Structure of stitch
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.39