The chain stitch is an embroidery technique where many loops of
thread are drawn together to make a chain-like pattern. You bring
the needle out at the top of the stitch line, and then holding the
thread below the stitch, insert the needle again INSIDE the
previous stitch (at the top of the stitch) and bring it out again
above the thread. This will form a chain.
For a broken chain, insert the needle again OUTSIDE the previous
stitch.