Basically the "official" storyline = canon.

For example. Take star wars, there are loads of books and games that advance the story in their own way. But some of the stories conflict with each other and some conflict with the movies and the books. That fan fiction you read where jar jar is a sith lord? Thats not canon because it is not the official storyline. That computer game where you play as the bad guy and kill luke skywalker? Not Canon, officially didn't happen. More of a what if scenario.

So Canon = official storyline

Then you get headcanon which you see people on reddit talk about. Headcanon = the official storyline in your head. I feel its easy to talk about this while we have canon nailed down (not that you asked but its related)

Head Canon = the storyline you accept in your head. Not the actual official storyline.

A great example of this is the mass effect series. The games ending was so awful that we were left lacking. Then i read this essay on what the ending really was and maybe we were all lied to and it was a plot twist from beginning to end. I personally prefer to think of the story in terms of the plot twist being true. It makes the ending satisfying for me. Its not the "official" ending as stated by the company who made mass effect but it is a better story like this. So this is now my headcanon. That fan fiction i read online as the ending in my own eyes is the ending.