"This is an apple."
The explanation of what Berniebud said, should you need it: http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit6/GramUnit6_3.htm
I'm not sure what you are trying to say, but the dash is optional in this case. In definitions, like this one, it's mostly used for clarity, so that the sentence didn't look like two words randomly thrown together, but it's not absolutely required. Not to mention that Duo mostly ignores punctuation anyway.