avant garde typically just means something that's "different" or not "mainstream".
wikipedia has a good definition which includes the following:
Avant-garde (pronounced [avɑ̃gaʁd] in French) means "advance guard" or "vanguard".[1] The adjective form is used in English, to refer to people or works that are experimental or innovative, particularly with respect to art, culture, and politics.
Avant-garde represents a pushing of the boundaries of what is accepted as the norm or the status quo, primarily in the cultural realm. The notion of the existence of the avant-garde is considered by some to be a hallmark of modernism, as distinct from postmodernism. However, this is not true in the case of music as many pieces are still being released which are generally considered avant-garde in popular culture.[2] Many artists have aligned themselves with the avant-garde and still continue to do so, tracing a history from Dada to the Situationists to postmodern artists such as the L=A=N=G=U=A=G=E writers in the 1980's to many art collectives in the "post-postmodernist"[3] times of today.[
