You have two ads in your sidebar - one with a more flexible placement, and one with a fixed, "sticky" placement. When you are looking at the breakdown of your ad units in the Mediavine dashboard, "Sidebar" refers to the first of the two placements; the one that is not "sticky".
It will render as either a 300x250 or a 300x600.
With the release of the Optimize for Desktop Pagespeed feature, and the overwhelming trend with our publishers always looking to do a bit more regarding site speed and user experience, you can adjust your sidebar elements to push your Sidebar ad so that it falls below the fold, and out of that first screenview.
To do this, you would want to take a look at your sidebar and the elements you have there. The goal is to add enough to push that first ad down and out of the first screenview so that it is loading "below the fold", while keeping your sidebar short enough to have at least 2 "sticky" impressions per session for site health.
Not sure or need help? Email us at firstname.lastname@example.org and we'll take care of things for you. Changes to your sidebar often require that we retarget ads anyway, so it's best to reach out.