Hello Bob, and welcome to Seven Forums.
A dynamic disc is basically like RAID, but using software only. It's really not used to much these days due to poor performance, and that you can't use it on drives that a OS (Windows) is installed on since it would render the OS unbootable.
Was there something specific you were wanting to use a dynamic disc for?
For more detailed information on basic and dynamic disks to help better explain, see the links below. Please feel free to ask any questions you may have.
Hope this helps for now,