用膳 is a formal, classical-style verb meaning 'to eat a meal'. It's used in historical contexts, by hotel or restaurant staff for a refined tone, or in ceremonial writing. You almost never hear it in casual conversation — 吃饭 is the everyday word. Like many two-character verbs in Chinese, 用膳 is a separable verb (离合词): you can insert measure words, modifiers, or time words between 用 and 膳, e.g. 用了一顿膳 ('had a meal').