Sidenotes keep context beside the text
A footnote makes you leave the sentence you’re reading, find the note at the bottom of the page, and find your way back. A sidenote sits in the margin, level with the line that refers to it, so a glance is enough. This is a sidenote. On a wide screen it sits in the margin to the right of the column.
In a note, write sidenotes as ordinary Markdown footnotes. Obsidian shows them as footnotes while you write, and the published note shows them in the margin. The syntax is [^1] where the note is referenced and [^1]: Your note. anywhere below it. Nothing else changes in how you write.
On narrow screens
When there’s no room for a margin, on a phone or inside a pane, a sidenote folds into the text. Its number stays in place, and tapping the number opens the note right below the line. Tap it again to close it. Edward Tufte’s books, and the Tufte CSS project, made sidenotes familiar on the web.
When to use one
Use a sidenote for something a curious reader might want and everyone else can skip: a source, a caveat, a small example. If the point matters to the argument, it belongs in the sentence.