Unit 5 text "legibility"

Actually, I’m in the “Build a Website from Scratch” course, but the issue is probably the same. In Unit 5, steps 6 and 7 mention that text can become illegible when the browser window is compressed. By “illegible,” what does that mean? I can still read everything: the window becomes narrow, but the text itself is still readable (until the window is compressed too much, at which point everything disappears).

Please post a link to the exercise you are referring to. Thanks.