Tree navigation is used to add markets manually to MARKETS grid view. You simply use tree to navigate to market that you want to add and once you click on it then it will get added to MARKETS grid view. After that bot will start monitoring it by your rules and get prices for it.
Navigation tree is automatically refreshed each hour, but you can refresh it any time you like by clicking on "Reload events tree" button.
Tree navigation is commonly used when you want bot to bet only on markets/selections that you manually pick or when you use ladder/grid interface for manual trading/betting.