Straight stitch is just a regular stich going in and back out, although a Lockstitch is similar to straight but the thread comes back around goes through itself to almost form a knot.
Answer: The lockstitch uses two threads, an upper and a lower. Lockstitch is so named because the two threads, upper and lower, "lock" (entwine) together in the hole in the fabric which they pass through. ... Then the take-up arm pulls the excess upper thread (from the bobbin area) back to the top, forming the lockstitch.
