Frames are an older HTML technique that split a single browser window into several independent sub-documents, each loading its own URL through the frameset and frame elements. Common in the late 1990s and early 2000s, they are deprecated in HTML5 and cause well-known problems for search engines.
The core issue is that framed content is not tied to one shareable, indexable address. A crawler lands on the frameset container but cannot cleanly associate the visible content with a single URL.
A corporate site built around 2001 kept its menu in a left frame and its articles in a right frame. Google indexed the frameset URL but could not rank the articles individually, because none of them existed at their own address. The fix was a straightforward rebuild: each article became its own page, with the shared navigation pulled in through a server-side include and styled with CSS. Rankings recovered once every piece of content had a real URL a crawler could reach.
If you need embedded external content today, a single well-managed iframe is acceptable, but the page's own primary content should always live in the main document. A technical SEO review will flag any legacy frames still blocking your content from being crawled properly.