A Ticket is the a collectible item that Rocket can find around Whoopie World. Collecting tickets is how progression in the game is made. There are 12 tickets, lettered from A-L, scattered around each world. The "L" ticket is obtained by collecting all the tinker tokens in that respective world.
Tickets are collected so they can be used to activate Ticket Switches, which require a certain number of tickets to activate. When activated, a ticket switch will open up a new world to explore.