A sticky item scrolls with the page at first, then stays near an edge of the screen when it reaches that point. It is like a bookmark that moves down a page and then holds its place while you read.
A section title or small navigation bar can stay at the top so people know what they are viewing. When the page reaches the next section, the sticky item can move along with it.
Make sure it does not cover the text people are reading. Sticky items need enough room on small screens and should not block buttons or headings.