口 is a pictograph of an open mouth — a simple square opening. You've already met it hiding inside 吃 (eat) and 喝 (drink): both have 口 on the left, because eating and drinking happen at the MOUTH. Now you know why.
口 is one of the most productive radicals in Chinese — any character about speaking, eating, or openings tends to contain it (吃, 喝, 叫, 唱). It also counts people: 人口 means 'population' (literally 'person-mouths'), a reminder that for most of history, a mouth to feed was how you counted a family.
A simple open square — an open MOUTH.
See 口 inside a character? It often signals the mouth's work: 吃 (eat), 喝 (drink), 叫 (shout), 唱 (sing), 名 (name — calling out in the dark).