A self canonical is a self-referential canonical tag: URLs with a self canonical therefore refer to themselves.
When and why are self canonicals useful?
A self canonical is useful when linking to URL versions with parameters or UTM tags. Without self canonical Google will index the parameters/UTM URL additionally. This leads to duplicate content. With a self-referral canonical, you communicate to Google which URL version is the "original" URL and should be listed in the index; the parameter URL version is then usually ignored.
As a best practice, it can be useful to provide all pages with a self canonical - even if the domain has no duplicate parameter URLs. In this way, you provide for parameter and UTM URLs that may arise later.