开口 is a separable verb (离合词) that literally means 'open the mouth' — it's used both for the physical act of opening your mouth and, more commonly, for starting to speak or finally saying something after hesitation. Because 开 and 口 can split, you will see forms like 开了口, 开不了口, or 开过一次口 when adding duration or ability. The word is often heard in social contexts where someone is reluctant to speak (e.g., 不好意思开口, 开不了口). The physical sense of opening the mouth (for eating, laughing, etc.) exists but is less frequent as a standalone verb.