Each portfolio element can display its description in 2 ways: docked or floating box. On the next screenshot you can see example of floating version. Floating box interacts with scrollign on your website. If you scroll down the box will slide out until it disappears. Afterwards you can access and click on portfolio items under the box. If you scroll from the bottom of the page back to its top, the floating box appears again. By clicking on the small arrow at the bottom of the box you will remove it completelly.
You can set up this feature via Page Builder in “advanced” settings of “portfolio element”. Settings you are looking for are displayed on following screenshot. This section is disabled by default and description box is docked above the portfolio gallery. This option is disabled also for all mobile devices with touch screens with respect to smaller screens or slower browsers.