Back stitch is a way to finish threading. The thread will not draw off without tie a knot if you use this method.
1.After stringing beads, back stitch to repeat the string line, then the string will not draw off easily. Then cut the string tail.
2.Example here: If you string to point A, it is finished basically. But you would like it look perfect, and do not tie a knot there.
3.Back stitch according to the red line and green line, then cut off the string tail.

Tutorial of back stitch