下结论 is a separable verb (离合词) meaning 'to draw a conclusion'. The verb 下 carries the action, and 结论 ('conclusion') is the object. In usage, you can insert words like aspect markers, measure words, or modifiers between the two parts, e.g. 下了一个结论 ('drew a conclusion'), 下什么结论 ('what conclusion to draw'). It's typically used in formal or analytical contexts, and it's common to hear 别轻易下结论 ('don't jump to conclusions').