iFrames are a HTML tag that allow you to embed other webpages inside your page in a small frame. They generally represent an older coding practice and are discouraged as they can complicate navigation, particularly in mobile, and are harder for search engines to index.
We recommend removing any iFrames if they don't serve a critical purpose, or could be replaced with more natural navigation. However, some coding libraries like Google Tag Manager may still rely on iFrames as part of their internal functionality to load external pages and code files, so you may need to evaluate your usage of them on a case by case basis.