When a cable lug is compressed over cable it is the accepted methodology to cover the connection location between them using heat shrink. Heat shrink is a tube, which looks rather like a bicycle inner tube, and is really rather self-descriptive! When you apply heat to this tube it shrinks in size, contracting to grip any item contained within. There are shrinkage tolerances so it is advised to size the tube as near to the item due to be sealed in advance.