诉苦 means 'to complain about one's hardships or grievances' — it focuses on pouring out personal difficulties or unfair treatment, often seeking sympathy. It's more specific than 抱怨 (general complaining) and unlike 投诉 (filing a formal complaint as a customer), 诉苦 is informal and personal. It's a separable verb (离合词): you can insert 了 or other elements, as in 诉了一晚上的苦.