The pickled Japanese plum often used in maki rolls is called umeboshi.
Umeboshi are salted, pickled ume fruits. They are commonly translated as Japanese plums, but ume (Prunus mume) is botanically closer to an apricot than to the European plum. The fruit is intensely sour and salty because of its citric acid and traditional salt curing, although modern varieties may be sweetened with honey or partially desalinated.
In makizushi, cooks may use a pitted umeboshi or umeboshi paste, often paired with strips of fresh green shiso leaves. Umeboshi is also widely eaten with rice, placed inside onigiri, or served as part of a breakfast or lunch. A single red umeboshi over white rice gives a bento the appearance of Japan’s flag; this is called a Hinomaru bentō.
The other options are different ingredients: shiso is an aromatic herb, gari is sweet pickled ginger served alongside sushi, and takuan is yellow pickled daikon radish. Umeboshi should also be distinguished from umezuke, which is pickled ume that has not been dried.