Backstitching is the act of reversing a machine stitch such that it overlays to lock the seam stitch together. Backstitching is only applied for a small portion, about 1/2", just enough to secure the seam ends thus prevent them from weakening or coming undone.
Conventionally seams should be backstitched both at the beginning and end of the stitch. Doing so will not only prevent the seam ends from unraveling while the garment is being constructed (especially when sewing crossing seams) but will also increase durability in the finished garment.
If backstitching both seam ends creates tension in the seam, one of the ends should be left not backstitched. You can release tension through this un-backstitched end by ironing in its direction. This is often the case with sheer, lightweight fabrics that tend to pucker under machine straight stitch.