Umeboshi Paste is a Japanese red paste made from puréed Umeboshi (pickled apricots.)
It can be bought in tubes or bottles.
To make your own, just mash Umeboshi.
When using Umeboshi Paste, bear in mind that it is very salty so hold back on other salt.
Often called in English “Umeboshi Plum Paste”, though the Ume fruit from which Umeboshi is made aren’t really plums.