Oh hey Larry the endosymbiotic theory is very complicated so I will have to break it down for you.
Lets start with Eukaryotic cells... Eukaryotic cells are cell that contain a nucleus and organelles and are enclosed by a plasma membrane. Look below for a diagram of a Eukaryotic cell.
The mitochondria and chloroplast are important organelles in the Eukaryotic cells. But the theory suggest that those two important organelles were once independent prokaryotic cells that were incorporated into larger host cells.
Thank you for explaining the endosymbiosis theory to me I understand it now.