Is this about Home Sharing movies, tv shows, music, photos, home videos, something else?
Folders could best apply to photos, so I’ll assume that for now.
When activating Home Sharing (File﹥Home Sharing﹥Turn on Home Sharing) in the iTunes application, you can configure where photos should be shared from (File﹥Home Sharing﹥Choose Photos to Share with Apple TV). [Home Sharing has to be turned on first.]
In the window that appears, select Share Photos From, then choose a source from the pop-up menu (Choose folder…).
Next, make a choice for either “All photos and albums” (from within that folder), where subfolders will be treated as albums, or: “Selected albums” to share some subfolders (now), but not all.
If the parent folder has no subfolders, then select albums may not be applicable.
Videos in the photos folder(s) will be ignored for Home Sharing to Apple TV. Sharing videos comes from the iTunes Library directly instead.
If you have multiple computers in your home network that do Home Sharing, then verify that each computer has a unique name for its iTunes Library.
For other (compatible) types of media than photos, the iTunes Library database is shared instead, regardless of where the file is located, as long as iTunes knows where and has access to it. So add other media to the app’s Library, either through the app’s menu or through drag-and-drop.
Use Home Sharing to share media from a computer to other devices - Apple Support
Stream content to Apple TV with Home Sharing in iTunes on PC - Apple Support
Play movies on Apple TV from iTunes on PC - Apple Support