Iframes are where you put a website inside your website. This can be useful if you want to embed a YouTube video on your website, and thats it.
Having iframes on your site is just dumb and not helpful; putting a YouTube video inside your website might look cool, but it adds so much clutter to your website. This can easily be fixed by just making a link to a YouTube video, as this does not take up huge parts of the website.
Putting an iframe on your site can also be bad because of the amount of tracking that can happen. The page is instantly loaded so the iframe can find out the domain that the iframe is in. The iframed website can also fingerprint the user in any way they want, as they can run JS on your site. If you want to see how trackable your site is, you can use Cover Your Tracks by the eff.
Iframes are almost never used in websites now. When iframes first came out, they were used largely because you could buy a domain name that was like a bank and then use an iframe to go to the bank. Security was not amazing back then, so the website that hosted the iframe could see all the data that went through it. Now that is fixed, so websites hosting the iframes can't see the data going through.
Iframes just complicate the web, as a website is not going to look the same everywhere because the two websites will have different CSS, so the sites will have different looks and feels.