는 is the topic particle.
For sentences where the verb can take an object, 는 and 가/이(subject particle) can both be used, with meaning depending on the context. The man eats an apple = Man는/가 apple를 eats = 남자는/가 사과를 먹습니다.
For verbs that can't take an object and more complex sentences, which particle you use matters more. 사과가 있습니다 = there is an an apple.
남자는 사과가 있습니다 = Regarding the man, there is an apple = the man has an apple.