WebDAV is an open protocol on top of HTTP which allows user agents to manipulate media files. It is used by cloud storage providers to let users manage multimedia files remotely.
Piktures is able to manage, view, upload to, and download photos from servers supporting the Webdav protocol.
Some cloud storage providers such as Box, pCloud, or Yandex support Webdav natively and it is how Piktures access your media files stored on those services.
As WebDav is an open protocol, it is also supported by personal cloud solutions. For instance, you can install your own instance of Nextcloud and access it through Webdav. You can also use your own hardware from Synology and configure Piktures to access it via Webdav.