When the child grow in your tummy, more pressure is exerted on the cervix. If you cervix is loose, the increased pressure would progressively dilate your cervix & you would end up to have a premature delivery => the baby just slips out from your cervix!
Premature baby has a lot of problems.
What the Dr. said is that he can sew up your cervix by thread so that your cervix could withstand the pressure arised from your growing baby's weight. When your term is due, then they would cut the thread & let you have a spontaneous labor.
I had friend who had done it before. It is quite simple & safe. But the mother has to lie in bed very often to avoid over-pressure to the cervix exerted by gravity.