Interest is an increase in price*. Lower interest isn't a discount, it's a smaller increase. Calling it a discount is like punching you instead of stabbing you and calling it nonviolence.
*In exchange for better payment conditions, but that's not relevant to the point.
There aren't any square brackets.
The form "function(){content}" in bash defines a function called "function" that, when called by name, executes "content". This forkbomb defines a function called : (just a colon) which calls itself twice in a new subprocess (the two colons inside the curly brackets). It thus spawns more and more copies of itself until it overwhelms your processor.