What is the word that describes the middle part of a presentation?
The body.
Merriam-Webster:
the main, central, or principal part: as (1) : the nave of a church (2) : the bed or box of a vehicle on or in which the load is placed (3) : the enclosed or partly enclosed part of an automobile
eg.
The body of my presentation consisted of me showing the audience photos I'd taken during my vacation, and explaining what camera filters I'd used. In my conclusion I suggested that they too should visit New Zealand.