The copula indicates the predicative relation between subject and predicate. In “Zayd is standing,” “is” relates standing to Zayd in an affirmative judgment.
«زيد هو قائم»
Zayd is standing.
In this Arabic construction, “هو” makes the connection explicit. Arabic may omit an overt copular expression, as in “زيد قائم,” while the affirmative relation remains understood.
The copula does not contribute a new defining property; it indicates the act of predication.