It's not really like a msgbox but you could call it a sort of msgbox because it does show messages. It is however modeless and without seeing the Java code for the default toast message, I would think it's very similar as much as it can be in B4A.
It is a panel with a label and set to the front most position with a timer to control the amount of time it is displayed. The issue is once the app using this class is released, the toast is released with it. The OS is what normally handles the toast and why it can still be displayed when you exit the app. That is the one thing this will not do, nor can it display as the top level form for a different application. But, it will do some things the system toast message will not!