"Segment" is an easy way of "numbering" your slides. It allows you to input a number on each slide, for easy creation of a list containing regular slide types.
This is how you turn on/off the segment number:
Click on the slide, to open the side menu
Open Segment section
Turn on/off the segment
Type the segment number (only numbers are accepted in this field).
Choose if the segment should be faded in and out or just in or out
Choose the segment number color.
Choose the segment box color.
These settings allow you to chain together multiple slides with the same segment number without animating it in and out on every slide (choose fade in on the first slide in the segment and fade out on the last slide in the segment).