A deferred deep link sends a brand-new install to a specific screen inside your app instead of dropping them on the home page.
A normal deep link only works when the app is already installed. A deferred deep link handles the harder case: someone taps your ad, does not have the app yet, goes to the store, installs, and on first open lands exactly where you sent them. The deferred part is the system holding on to that destination across the install gap and delivering it once the app finally opens.
Say you run an ad for one specific pair of shoes. Without a deferred deep link, a new user installs and opens to your generic home feed, has to hunt for the shoes, and often just leaves. With it, they open straight to that product page. Open your onboarding funnel and compare first-session conversion for deep-linked installs against cold ones; the gap is usually big enough to notice.
Deferred deep links lean on the same attribution matching that privacy rules have chipped away at. When the match is fuzzy, the wrong destination or no destination gets delivered, so you need a sane fallback screen for the installs the link cannot place.
Send a new user to the thing they tapped for, not to your front door.