Kiln dried lumber is put into a large oven and force dried down to approximately 12% moisture. Green lumber is lumber still holding sap moisture from the field. That could be a moisture level of 20-30% moisture or greater depending on where it came from. This type of question should come up when attempting a project with any joins, 45 degree cuts and miter cuts. The problem being when the wood is holding moisture it will have a tendency to shrink and create gaps in your joins and miters. This will create water penetration points and cut down the longevity of your project.