开饭 is a separable verb (离合词) meaning 'to start a meal' or 'the meal is ready'. It's used when food has been prepared and it's time to eat. You can separate the two characters to insert a measure word or duration: 开了饭 (started the meal), 开过饭 (have had the meal), 开两次饭 (start the meal twice). It's a common household expression — the host or cook announces 开饭了 to call everyone to the table.