Canon just means accurate to the lore. Be it a movie/show/book/comic ..etc

Non canonical is the opposite, where it hasn't been said/isn't specifically something that happened or was part of the content. Like fan fiction, art, or other forms of narrative not written by the creator/team/group the content came from.

Think of things like star trek. It is canonical that data is an android devoid of emotions. But it is not canon to say he was does feel, if all he has done is learn about the process. Some fanfic will have it so he has a update to his OS that allows it, or similar, and that is not canonical.

Head cannon in relation to it all is a fans own idea of what is true/right in that fandom or setting. Like some have it as a head canon that the characters reality is all a dream during a coma experienced by the protagonist. It may become popular enough that others agree and follow that head canon, but it still isn't canon to the original.