The article will often stand without a noun (an “orphan article”). In such cases, it takes on the nature of a pronoun and always implies some person or persons previously mentioned.
στραφεὶς δὲ ὁ Ἰησοῦς καὶ θεασάμενος αὐτοὺς ἀκολουθοῦντας λέγει αὐτοῖς· τί ζητεῖτε; οἱ δὲ εἶπαν αὐτῷ ῥαββί, ὃ λέγεται μεθερμηνευόμενον διδάσκαλε, ποῦ μένεις; (John 1:38)
Notice there is no noun that corresponds to οι. This word then becomes “they” and its antecedent is John’s two disciples (John 1:37).
