Translation:I will eat dinner.
It's the difference between "simple present" and "continuous present" tense, in both English and Japanese. Here's a good summary of the differences in English.
食べます is also simple present tense, while 食べています (as @KiritsuguZFC points out) is continuous present tense.
Be aware that the usage of either tense in Japanese doesn't always line up with English usage though.
"Dinner" originally meant "the main meal of the day" and is still used that way when referring to (say) "school dinners" which are always midday meals and very much "dinners". But for some regions/classes "dinner" is the evening meal and hence it becomes specialised to that purpose, displacing tea/supper.
One side of my family would eat: breakfast, lunch, tea and supper and then occasionally have dinner as an alternative to one set of those. While not entirely historically accurate, think of that as the proto-system, and then assume that everywhere in GB there is some specialisation of that system down to three meal names.
The kanji for ゆうごはん is 夕御飯, where 夕【ゆう】means "evening". My dictionary also gives me 晩【ばん】= "evening" and 夜【よる】= "night, evening".
To my understanding, there's a fair bit of overlap between them and even to native Japanese people, it may be rather ambiguous, but if I had to compare them:
- 夕 is the earliest, more like "dusk" (e.g. 夕陽/夕日【ゆうひ】= "sunset")
- 晩 would come next, covering early evening (e.g. 今晩は【こんばんは】= "good evening (opening greeting)")
- 夜 is the most general, covering all night, but especially later evening (e.g. 深夜【しんや】= "late at night", 夜中【よなか】= "middle of the night")
Japanese has past and non-past tense. But there isn't a dedicated future tense.
The non-past tense can be used to describe habitual actions "I get up at 7am everyday" or future actions "I will get up at 7am tomorrow."
(Mainichi, gozen 7-ji ni okimasu)
(Ashita, gozen 7-ji ni okimasu)
Without context or a time reference, the verb could be translated as either "I get up" or "I will get up."
I think it's more about teaching you how to form sentences of your own based on previous knowledge in the language. Being able to say "I eat dinner" is ineffective at the start; most people eat dinner. It's ordinary. But knowing how the sentence works is a useful skill to have and teaching people odd sentences like "I eat dinner" is a good way to go about it.
The difference between using を and using は doesn't change the meaning, so much as it changes the emphasis of the sentence.
Essentially, there are three things in this sentence: the subject "I" (which is implied), the verb "eat", and the object "dinner". By using the object marker を on "dinner", you're simply saying the verb happens to it, and your subject remains the most important thing in the sentence. When you use the topic marker は instead, you are making "dinner" more important than the subject, so the emphasis is on "dinner" instead of "I".
The following are not accurate translations, but you can kind of think of:
- ばんごはんを食べます as "Look at me, I eat dinner"
- ばんごはんは食べます as "Dinner! I eat it"
We have always been there, you just didn't know it yet.
食べます = non-past
"I eat" (routine action)
"I will eat" (planned action)
食べました = past
"I ate" (past action)
食べています = present progressive
"I am eating" (current action/state)
Please be aware, due to differences in the language, verb tenses do not always map across from English to Japanese perfectly. The dictionary form of verbs can be used in a similar way to the simple present tense in English and te-iru form can be used in many situations that would call for present progressive form in English. But it would be a mistake to assume that it is always a 1:1 conversion. Certain things just do not work the same, because the underlying language concepts are categorized differently.