The system prompt can be used to describe to the AI what the world rules are. For example, it is always cold.
The dictionary, as far as I can tell, is a way to explain some words to the AI that it normally wouldn't be familiar with. For example, if you want to create your own race for a character with a custom name, you can set the name as the "key" and then explain what the race looks like and what they do inside of the "value".