A pop-up is a window that appears over a page's content, most often an ad, an email signup form, or a discount offer, either immediately on arrival or triggered by a specific action like scrolling partway down or attempting to leave the page.
Google specifically penalizes what it calls "intrusive interstitials" on mobile, pop-ups that cover the main content immediately after a visitor arrives from a search result, since they actively work against the experience the visitor came for. That penalty doesn't apply to every pop-up; legal or age-verification notices, reasonably sized banners, and pop-ups triggered later in a visit (rather than blocking content on arrival) are generally treated differently. The practical guidance that's held up regardless of the exact rules: a pop-up with a genuinely easy-to-find close button, that doesn't block content the moment someone lands from search, and that appears at a moment that makes sense (after they've actually engaged, not the instant the page loads) avoids most of the harm, both to rankings and to the visitor's actual experience of the page.
