进餐 is a formal verb-object compound meaning 'to have a meal' — it's used in written or polite contexts, like restaurant announcements or formal invitations. As a separable verb (离合词), the two characters can split when you add a measure word or time expression: 进过餐 ('have had a meal'), 进一次餐 ('have a meal once'). You wouldn't normally use it in casual conversation (吃 is more natural there).