What does Adeste Fidelis mean in English?

"adeste": comes from "adsum, adesse, adfui, adfuturus" (to be present);

The imperative of "adsum" is "ades" (2nd person singular) and "adeste" (2nd person plural).

So it means "Be present", or "Come!", as in "Come, all ye Faithful," in the carol, "Adeste, fideles!"