The "Spending Limits" feature is a way to require a predetermined amount to need a PIN to spend.
For example you can set the spending limit to $100 which will then require a PIN to spend $100 or more from that wallet. Note this can also be set to zero, meaning each transaction will require the PIN.
This can be enabled by tapping Menu > Spending Limits > then entering your password and toggling the feature on. You can input any amount you would like to require a PIN