Here's a mini science lesson for ya'll.
Carbohydrates are the main energy source for our bodies. When we eat carbohydrates, they are broken down into mainly simple sugars, early on in our digestive tracts, in which our bodies can use as an energy source. A carbohydrate that behaves this way in the body, is an available carbohydrate.
However, not all carbohydrates are broken down early on to provide our bodies with a direct energy source.
Some types of carbohydrates make it all the way to our large intestine which are either fermented by the bacteria that live there (which technically can provide your body with a small energy source) or passed through to the toilet. These types of carbohydrates are called unavailable carbohydrates.
So very (very) simply put, available carbohydrates provide you with a readily available energy source and unavailable carbohydrates don't.