The Sidebar First layout changes the position of the sidebar. On desktop view, it will appear on the left, and on mobile view, the sidebar will be at the top of the page.
On mobile, viewers will see the collapsed navigation menu at the top of the page, below the title and breadcrumb trail. You'll see that if you're viewing this page on a mobile device now, or if you resize a desktop browser window to a narrow width.
One cautionary note, however - this layout change will position all sidebar content above the main content in mobile view, not just the sidebar navigation menu. So if you're including additional secondary content in the sidebar (event lists, related links, etc.) the Sidebar First layout may not be the best choice.You can apply this selectively to an individual section, and set all other sections in the branch to use the standard Site Specific layout. Here's how to set that in the section's Page Layout properties.