Lockstitch – a sewing machine stitch formed by the looping together of two threads one on each side of the material being sewn. — Merriam-Webster
This video was produced by Threads Magazine and published to YouTube on March 11, 2015. It shows how a lockstitch is made.
Now I'll show you how a stitch is made. OK let's identify a couple of these key parts. This is the take-up lever, it moves back and forth. It releases the tension on the thread so that the bobbin hook can grab it and make a loop that goes around the bobbin. This is the tension. This is the bobbin.
These are the fabrics going through and these are the feed dogs that are moving the fabric through. Each one of these is a stitch. The take-up lever brings the thread down as the needle plunges into the fabric. This releases all the tension on the thread so that the bobbin hook can grab it and form a loop that goes around the bobbin thread. When that loop goes around the bobbin thread, the take-up lever moves up as the needle is coming up and it tightens the bobbin thread up underneath the fabric. You see how the feed dogs are pulling it through? That cycle happens for every stitch and the timing and the tensions are totally important.