In a scientific sense amorphous means lacking a crystalline structure, something without solid form. In a broader sense, the word describes anything that lacks a distinct shape or organizing theme, be it a work of art, a political movement, or even someone's direction in life.
The Latin roots of this word are clear — morph means form and a- means lacking. Lacking form. When used to describe creative works or ideas, it usually refers to a lack of organization and would rarely be considered a compliment. If your writing is amorphous, it's probably not fully formed. An amorphous crowd of people seem to be random and unconnected, until they all start dancing the macarena in unison.