It's the predicative instrumental case.
Let's take the phrase "Они сделали жизнь интересной" (= They made life interesting ) as an example. In this phrase, "они сделали интересной" (they made interesting) is a predicative expression and "жизнь" (life) is a direct object (in the accusative case). Since the adjective is a part of a predicative expression and the noun is an object (i.e. they are different parts of the sentence) they shouldn't agree in case. The adjective or participle as a part of the predicative expression takes the instrumental case. It doesn't really have any semantic meaning, you just need to remember it.