What does Top Stitch mean?

A simple stitch that is straight and very common.

Heddels explains Top Stitch

Topstitching is a sewing stitch usually used for hems, seams, and necklines to give a garment a more finished appearance.

Both functional and decorative, it is sewn on the right side of the garment and helps to keep layers in place and edges lying flat.

Additional Resources

Topstiching on a pair of Cheap Monday denim: