I think it came from this or did it come from win10 didn't look.
Anyway its what was used to be a half checked box, usually found in option dialogs and the like, if things are half checked it means that parts of a setting or group of options are not selected.
You can hit space twice to check it all or uncheck and recheck but sometimes have to right arrow to the list and check everything you want.
In some cases such as some windows settings like for examples the turn features on and off section it could mean part or none of an option is checked, but usally means that something installed part of one thing as a requirement but not everything.
You don't need everything on your system necessarily but still its your choice what you do with the checkbox.
The other place is where multiple atributes say in files exist, ie partly hidden or partly read only.
In 99.9% of the cases, you don't need to fiddle with hidden or readonly files, and in most cases windows will change atributes as you open and save files depending if they are yours, which can be set in the security tab however please note you may have to turn simple or use sharing wizzard off in folder options to access this.
Or if you need something windows can change on the fly without the user knowing as long as the files are yours on your account then thats ok.