哟 is a casual spoken interjection of mild surprise, common in everyday northern speech and avoided in formal writing. It also serves as a soft sentence-final particle, often toneless, marking exhortation or filling out a song line, as in the work-chant 嗨哟 (heave-ho).
Mouth radical on the left. Interjections come straight out of the mouth, so 口 marks practically every spoken-particle character (啊, 呀, 哎). The radical is a giveaway that the character represents a verbal or exclamatory sound.
约 supplies the sound, with the rime simplified to yō. The phonetic match is approximate but the initial y- and rounded vowel are preserved. 约 here is the simplified form with 纟 silk on the left and serves purely as a sound clue.
