I am simply roughly speaking to bring in a bubble gown for the first time? On the pattern where on earth it tells you what stuff...

On the pattern where on earth it tells you what stuff you need I come across underlining (Sew-in interfacing). This is for the bodice piece. I'm not really sure what it means and want to grasp it right. Could someone who does dressmaking enlighten me please.
Answers:    When it say to use interfacing it means you hold to make the d¨Ścolletage line firm that mechanism cutting an interface to hold on to the neck queue firm and in shape .You use inter facing on lapel and cuffs .and down the front when you are putting in button holes .Hope you can below stand what iam meaning.