It is a small plastic bowl filled with pink custard. Normally, the custard is drank through a straw built into the bowl, though it can also be eaten directly through other means.
Collecting the custards is the main objective of the saga. The goal of many of the games is to collect all of the custards that are located on the area, winning when all of them are collected.
Since Slendytubbies II, the player was able to select the amount of custards, the maximum being 25. In some of the games, enemies would patrol near the custards, usually staying at the one closest to the player.
The games' events begin with all four of the main Teletubbies eating infected Tubby Custard, without knowing they are infected. Due to this, Tinky Winky starts to become a monster and kills the other three afterwards, creating the Tubby Massacre.
As the other three had ate custard beforehand, they later turned into monsters and traveled to different areas. Several of them had transformed into larger monstrous forms due to the custard, as Tinky Winky becomes huge and muscular after eating an extra custard. There also had been other teletubbies who ate infected Tubby Custard and became monsters nearby. Additionally, many Tubby Custards had been scattered around various areas, presumably by Noo Noo, as he is shown to be the one who had infected the custards in the first place.
The Guardian is trying to collect all of the custards so that he may research them later, possibly for a cure.