A stitch is formed when using a sewing machine through a combination of the machine's needle and bobbin thread.
When the sewing machine is turned on and the foot pedal is pressed, the needle moves up and down rapidly, piercing the fabric and pulling the thread down through the fabric. At the same time, the bobbin thread is pulled up through a loop in the needle thread, forming a tight knot called a lockstitch.
This lockstitch is what holds the fabric together, and it is formed by the tension between the needle and bobbin thread. The sewing machine's tension settings can be adjusted to ensure that the needle and bobbin thread are pulling evenly, creating a strong and secure stitch.
Different types of stitches can be formed by adjusting the sewing machine's settings, such as the length and width of the stitch, the tension of the thread, and the type of needle being used. With practice and experience, a skilled seamstress can create a variety of different stitches using a sewing machine, from basic straight stitches to more complex decorative stitches.