A new slide is started for each level-1 section, i.e., .SH 1 and .NH 1. (Note of course that it doesn’t make sense to convert generic groff documents into presentations. The sections have got to be pretty short!)
An HTML page meant for presentation uses larger, bolder fonts by default (but you can add your own CSS). There are no navigation bars between slides.
Consult the Slidy documentation for the keypresses and clicks you can use to progress through or dynamically alter the look of your presentation.