If multiple posts are set as sticky, the most recently published of the sticky posts will be displayed. Sticky posts are displayed twice: once in the top of the posts list on the blog page, and again in their actual position in the chronology. Most WordPress themes include some sort of marking, like a text label or an icon, to indicate that a post is sticky.
To mark a post as sticky, go to Posts → All Posts and click the post to open it in the edit view. Locate the “Publish” meta box in the top right corner of the screen, and click on the blue “Edit” link next to the “Visibility” text. When you click it, a list will expand. If your theme supports sticky posts, a checkbox will be displayed in the list with the label “Stick this post to the front page”. Check the box, click the “Ok” button and then click the “Update” button (or “Publish”, if it is a new post). Your post will now be displayed as sticky on the site.