"spiser" = eats, e.g. the child eats. This can mean the child eats, once, now. This can also mean that from time to time, the child eats. In English, the grammar gives you no clues which meaning is the right one. We work it out from context.
"spiser" = is eating, e.g. the child is eating. This means the child is eating, now, has already started eating and is continuing to eat.
So, "the present tense" can mean three different ideas of time: a single action, an intermittent action and a continuous action.
In English, there two different ways of expressing these three ideas of time: "eats" and "is eating".
In Norwegian, there is one way of expressing these three ideas of time: "spiser". The grammar gives you no clues which idea of time you are supposed to understand. We work it out from context.
If you are given "spiser" in Norwegian, then you can translate it as "eats" or "is eating" and it doesn't matter which way you choose. Both versions are a correct translation.
Hope that helps.