"Movie Clip" is an option for symbols that is really only important when you want to export stuff to use in Flash/Animate CC.
"Movie clip instance" is a name you assign to an instance. An instance is a symbol instance plaed on the artboard.
In case you don't want to export to Flash/Animate CC, just ignore this.
In case you want to export to Flash/Animate CC: learn about object oriented programming and/or have a serious talk with your web developer, because you will need it for your workflow.