The Brain is where you can adjust how much Plum will deposit for you, according to your preferences.
This is where you'll find the Rules:
Rainy Days (Plum Pro)
52-week challenge (Plum Pro)
Also, it's the place to set if you'd like Plum to set aside for you in Overdraft or now by tapping on the Overdraft Deposits menu.
Finally, you can find the Splitter, which is a feature that allows you to, well, split the auto-deposits made by Plum to various Pockets.
The Brain, Rules, and Splitter can be found on the Homescreen right after logging in.