yes; or, that's how it is; or, an interjection for making an exclamation (see Molina)
Alonso de Molina:
ye. si, o asi es.
ye. o, interjection para hazer exclamacion. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 34v. col. 1. Thanks to Joe Campbell for providing the transcription.