The author is too young to remember frames, huh? :) I'd just link the Wikipedia article ( https://en.wikipedia.org/wiki/Frame_(World_Wide_Web) ), but I feel like it doesn't explain it very well.
So, to briefly explain -- in the early web (OK, not the really early web, but once it got popular) frames were very common. They were commonly used for menus/sidebars, as well as things like headers that were meant to stay visible. So perhaps a page would have three frames -- a frame up top with a persistent header, one on the left with a sidebar, and then one main frame with the content.
Each frame acted fairly independently, with e.g. its own back/forward history (a link in a frame would by default only affect that frame, although you could have links in one frame open in another, useful for implementing a sidebar). This meant using back/forward/reload/etc with frames could be pretty troublesome and unintuitive. If you somehow got a page into a messed-up state where the wrong thing was loaded in one frame, it could be hard to get out of this state other than by navigating to the page from the start. Or it could be troublesome if you accidentally loaded just the content page without the framing page it was supposed to be a part of.
CSS got rid of most of the use case for frames, which is good, because frames were a real pain to deal with as a user!