display: none is a CSS property value that removes an element from the page layout entirely. The element still exists in the page's HTML. The browser has loaded it. But it takes up no space and is completely invisible, as if it had never been placed on the page.
This is different from visibility: hidden, which makes an element invisible but preserves the space it occupies. With display: none, the element is gone from the layout. Nothing shifts to fill its place because, from the layout's perspective, there's nothing there to fill.
It gets applied unintentionally in several ways. A theme stylesheet may hide elements with a particular class at certain viewport sizes. A plugin may inject a rule that matches a broad selector and catches elements it wasn't meant to catch. An animation plugin sets it as an initial state and the animation never fires. In all these cases, the element is in your page editor, your settings show it as active, and CSS is the only reason it isn't visible. Loupely Lens identifies the specific rule applying display: none, where it came from, and whether it's on the element itself or inherited from an ancestor.