If you could give me an example of other sentences I could elaborate, but in this case, specifically, it is an impersonal sentence. This sentence literally means: [in the garden] an action [playing] is being performed. A good way to translate this into English would be: "There is playing going on in the garden." Doesn't make the best of sense in English, but you get the idea.
This unit is going to be a nightmare. We can all see what the Dutch means but the sentences are untranslatable. So my 'paraphrase', 'The garden is used for play', is marked wrong, but the 'correct' answer is not English in any shape or form. I may have to abandon this unit because of the frustration, which is a pity as the topic is very important.