It's the share of your phone visitors who, after tapping a button on your Link, landed inside the music app (Spotify, Apple Music, YouTube and so on) rather than on the app's website. If 100 people tap through from their phones and 80 of them end up in the app, you'll see 80%.
Why it matters
Someone who lands in the app is already logged in and one tap away from saving your song, following you or adding you to a playlist. Someone who lands on a web page usually doesn't have the app installed, and those visits rarely turn into a listen that sticks. So two Links with the same number of clicks can have very different numbers of real listeners behind them.
What's normal?
The number is really a measure of how many of your visitors have the app installed, so it moves with your audience rather than with anything on your page. A few patterns we see across all Links:
We already send visitors straight into the app whenever their phone allows it, so there's nothing for you to set up.
Where to find it
Using it with ads
If you're running ads, a country with cheap clicks but a low "Opened in app" rate is costing you more per real listener than the headline price suggests. Worth knowing before you put more budget there.
A few notes