What does decomposed mean?

Decompose means it has broken down into fluids and matter. For example, a potato will get mushy as it decomposes. Decomposition describes the state of decomposing.
To be decomposed is to have broken down into separate parts. We use the term "decompose" to describe objects that have been left to rot, such as organic materials. For an exact term, and other terms, try this: http://webster.com/