It means the site is saved to your home screen (on mobile) or taskbar/dock (on desktop). When opened that way, it appears in a window with minimal browser chrome and with the site's icon and branding instead of the browser's, so it somewhat resembles a native app (even though under the hood the browser is still doing all the work). Sites that offer this will often also use a Service Worker to remain functional in the absence of an internet connection, since native apps are usually expected to do that; these and some other web APIs are often conceptually bundled together under the term "progressive web apps", originally coined by Google. See https://developer.mozilla.org/en-US/docs/Web/Progressive_web... for more.

I would expect web developers these days to mostly be aware of this, since it's been in widespread use for a while, so I didn't find it odd that the article assumed that level of background.