Translation:It is said that the watermelon is delicious.
I would like some clarification on this myself. Above if you hover over Λέγεται it gives "they say" as an alternative, but that doesn't sound passive at all. The wiktionary article on λέγομαι (and it's conjugated forms) translates it as "i am called" which fits more with the passive voice.
In English the sentence seems unnatural, like there's some specific watermelon renowned far and wide for tastiness, somebody ate a few bites but decided it was too good to just eat, and now people travel vast distances to visit the famous melon and get a selfie with it.
Is this really the intent? Or might a Greek speaker say this in order to mean something more like, "It is said that watermelons are delicious," referring to watermelons in general?
As @Rich717872 so correctly say there is no specific intent on the part of o the course nor does it have any metaphorical meaning. You can read a lot of context into this sentence but it's just a sentence to teach vocabulary and syntax.
If it were plural it could mean "watermelons in general".