SplashTix is a system that stores merchandise and food credit on your ticket. When you checkout at a concession or retail stand, your ticket will be scanned and the value will be removed from your ticket. (Note: not all stands accept SplashTix credit)
Not all tickets come with SplashTix credit pre-loaded. Most often, season ticketholders in select locations have SplashTix included with their season-ticket package. If you purchase tickets from one of these seatholders, the credit will be transferred to you. Typically, you would see these in the seller's notes when purchasing tickets.
All SplashTix credit expires once the game is over, so make sure you use it before you lost it!