Cartouche is an enchantment subtype introduced in Amonkhet, which only appear on auras.
The five cartouches connect together
Cartouches enchant creatures, giving them a boost that represents some kind of success or accomplishment. They give the creature they are enchanted to a boost of +1/+1 and a keyword, which is primary in that color. They also carry a secondary enter the battlefield effect, which also directly reflects the flavor of its color.
Amonkhet cartouches are linked to the Trials of the Five Gods and each Cartouche shares a part of its name with the trial corresponding to its color. However, in flavour cartouches are also used for control magic, such as maintaining the Anointed under control as well as restrain dissenters.
In the Hour of Revelation, the God-Pharaoh's angels stripped former initiates of their cartouches and the fortifying magic they provided.
All trials have and enter the battlefield effect and a secondary effect which returns them to their owner's hand when a Cartouche enters the battlefield under your control.