It's probably the same as in English.
In case of "Это яблоко" = "This apple" more stress is put on the pronoun "Это".
In case of "Это яблоко" = "This is an apple" the entire phrase is pronounced neutrally / evenly, without stressing / emphasizing.
I was under the impression that articles are not used in Russian. As such, I translated this as "This apple". I guess I am taking the no article thing too literally, when it is only taken into consideration for the writing and speaking of the language.