Basic Action: When a bet is boxed, the horses selected can come in any order. So, if you like the No. 3, No. 4 and No. 6 horse in a given race, you could box them in a trifecta. If those three horses finish in the top three positions (in any order), you’ve got a winning ticket.
The “key” is essentially the older brother of the “box”. Rather than boxing all of your horses (where you win if they come in any order), keying allows you to more boldly back up your convictions about a certain horse by locking the horse into the top spot, and then using the other horses you sort of liked in the race “underneath” in your exotic bets.