"In the words 'The Lord is One' there are many meanings of the term 'One.'
'One' does not only stand as against dualism (or trinity, or any other kind of plurality of gods).
It also includes God’s one-and-onlyness, in the sense of “there is none else besides Him” - (Deut. 4:35).
Compared with the 'truth of His existence,' no other reality counts."
From "The Portion of "Shema" (Deuteronomy 6:4-8)" by Rabbi Adin Steinsaltz