Translation:I eat dinner at seven P.M.
をis the direct object marker. When you use an "action verb" (aka a transitive verb), you mark what is receiving the action with a を. So for example, "eat" and "drink" are transitive verbs.
In this specific example, ばんごはん (dinner) is the thing that is receiving the action - the thing that you are eating. So you mark it with a を after it.
Another example would be "I drink water." Water is the thing that you doing the action - drinking - to, so you mark it with a を. In Japanese, お水をのみます。