A line type was historically a way for distributed elements layout to display based on the process being used. They are usually 2 or more layers with offsets from each other. Below is a 3D view of example of such a line.
This is common in MMIC design where multiple metal layers are combined to provide lower loss or higher current density capability.
Designers like to draw single shapes and them draw with the line type layers. This was possible previously by drawing a shape and then changing the properties to use a line type. Starting in V13, you can select the line type first as you are drawing shapes.
This video demonstrates how to draw shapes using Line Types.