Your Header Bar was set to Absolute Initial Position. The height of an absolute positioned bar is not taken into account in the page so the content below it goes up to the top of the page.
Since your just have a bar with solid white background, it's best that you set your Bar to Relative Initial Position.
Absolute Positioned Bars are usually used to achieve a Navigation Overlay Effect and the Header is usually assigned to specific pages that will use that effect instead of the bar being assigned globally.
For more details regarding the usage of Header and Footer Builder, please watch the video at https://theme.co/apex/forum/t/pro-header-and-footer-builder-introduction/103
Hope that helps.