Archaic honorific second-person plural pronoun. Used in historical or classical contexts to address multiple people respectfully. In modern Japanese, あなたがた or 皆様 are used instead.
To all of you, I express my deep gratitude for your efforts on this occasion.
Compound of 御前 (おまえ, 'you', originally honorific) + 方 (がた, pluralizing suffix). The exact historical derivation is uncertain, but it follows a pattern of forming plural pronouns with 方.