These two sentences can have different meanings in English:
The apple is green. (Meaning the apple is not ripe.)
The apple is green. (Meaning the color of the apple is green.)
In the first case, our example speaks of the condition of the apple. The apple is green because it has not yet ripened. When the condition of the apple changes, that is, when it has ripened, it will no longer be green, it will be ripe.
In the second case, our example speaks of the essential characteristics of the apple. The apple is green in color. This particular apple remains green even after it has ripened.
In English, the verb "to be" can be used to tell how something is (the condition) and what something is (the essence).
In Spanish, a different verb is used to express "to be" depending on whether the speaker intends to address a condition or an essential quality.
La manzana está verde. The apple is green. (condition)
La manzana es verde. The apple is green. (essence)
Note how the adjective "verde" actually changes meaning, depending upon whether it is used with "ser" or "estar".
To address condition, use "estar".
To address an essential quality, use ser
If you are talking about what something is, use ser; if you are talking about how something is, use estar.