They are Jews who have come to believe that Jesus (Yeshua) was indeed the Messiah promised to their people and prophesied in their Scriptures (the tanakh, or Old Testament)
This does NOT mean that they are “Christians” , though many Jesus-believing Jews do join Christian churches, and may cease to practice their Jewish faith. But the Messianics wish to preserve their Jewishness, rather than be “be assimilated”, though their adherence to Jewish observances like circumcision and kosher diet varies widely. Almost extinct as a religious grouping from the fourth century to the 19th cent., their numbers have exploded since the reconquest of Jerusalem in 1967, especially in Israel, the USA and in Ukraine.