"appje" is the diminuitive form of "app", which is a shortened form of "whatsapp" (referring to a whatsapp message).

In English, you would never shorten the phrase "I'll send you a whatsapp" to "I'll send you an app" because the word "app" would be confused with a mobile app.

The Dutch still use the word "app" to mean a mobile app; "appje" means a whatsapp. So the diminuitive form allows them to disambiguate.