What does ?attr/ mean on Android?

A style attribute resource allows you to reference the value of an attribute in the currently-applied theme.

https://developer.android.com/guide/topics/resources/accessing-resources.html#ReferencesToThemeAttributes

?attr/ references to attributes. Attributes are values specified in an app's theme. The attributes in your example are all values specified in the themes provided by the support library. Android also has its very own attributes which can be used with ?android:attr/.

The actual value that is going to be used in the end depends on the theme used to inflate the said layout. This theme can be specified in the manifest in the <application/> block for an app wide theme or in the <activity/> block for a specific activity. You can also override this theme during runtime by using a different context (see ContextThemeWrapper and LayoutInflater)

It is considered good practice to use theme attributes instead of hardcoded values in your layouts, as it allows for easy customization. For example, when you create custom views, you can use ?attr/colorAccent so that the user of the view doesn't have to provide a color, and it is going to use the colorAccent used in the app themes instead.

This becomes even more relevant today, as with the introduction of Dark Themes in Android Q, your layouts should specify an attribute so that the end value is different when using a light theme vs a dark theme.