A lockstitch sewing machine creates a stitch by interlocking a top thread and a bobbin thread. The top thread, which is fed from a spool, is threaded through a needle, while the bobbin thread is wound into a small spool that sits underneath the needle plate. As the needle penetrates the fabric, it carries the top thread down, creating a loop which the bobbin thread catches. As the needle ascends, the two threads interlock, forming a secure stitch. This simple yet effective mechanism is what makes lockstitch machines popular and indispensable in numerous sewing applications.