Translation:This movie was very interesting.
おもしろい is an 'i' or true adjective and it's ending changes to show tense and positive and negative - sort of like having です built in? na adjectives however like しずか or きれい need help to modify nouns ie. they can't modify nouns directly, な helps them to modify nouns. The endings of na adjectives don't change so です does that for them.
おもしろい can mean both interesting and/or funny. But funny as is in funny haha - a funny joke etc. The difference is that たのしい means fun as in enjoyable. おかしい (not to be confused with おかし which means candy/lollies) means funny as well BUT it means hilariously funny or funny in a strange way and it can also mean strange - as in funny in the head. My understanding is that おもしろい means interesting or funny - amusing - like chuckleworthy, whereas おかしい is tears streaming down your face funny.
Duolingo could be better. If no updates like this are made then it makes "learning" more confusing than engaging. Same goes with the leaderboards where clearly now, not the quality of your learning but your score only matters for any kind of lesson and language you take. For instance, if one is a proficient English speaker and in the lead on the scoreboard in English, while another has a lower rank, a proficient Spsnish speaker and learning French, what exactly does this mean?