In a strict sense, cultural appropriation is simply “the adoption or use of elements of one culture by members of a different culture.” (Wikipedia)*
However what we’re talking about here (and the more commonly applied usage of the term) is the misappropriation of elements of a marginalized culture by a dominant culture.
Check out the resources below to learn more.
*Actually, that Wikipedia entry provides a helpful overview of cultural appropriation and its related issues…I recommend it!