We call it the “sticky add-to-cart panel”, so when you scroll down the page past the point where the regular add-to-cart button goes out of view, this panel appears in view to help with sales conversion.
It should only be a narrow panel and shouldn’t take up half the page, but if you just want to remove it please add the following code to Appearance > Customize > Additional CSS:
#wc-sticky-addtocart.active {
display: none;
}