Probably the most common grammar in the Japanese language. A Copula links a subject with another concept to describe it. In English this is done with a verb (to be) in the form of "is", "am", or "are".
In Japanese, the copula is expressed by following an idea with だ, which links that idea with the subject (often an implied subject).
Japanese doesn't have articles like 'the', 'a' or 'an'.
The subject (it/I/we/they) is left implied here. Japanese doesn't always require explicit subject markers like english does. It relies a lot more on context.
Apple and student can be singular or plural depending on context, there is no distinction in Japanese.
だ can also be dropped in very casual settings.