What do the digits 3, 7 and 9 mean in a darija text?
In a darija message, digits stand in for Arabic letters that the Latin alphabet lacks. 3 is ع, a throat sound. 7 is ح, a strong breathy h. 9 is ق, a k pronounced at the back of the mouth. This writing is called arabizi. Once you know these three digits, most of your family's messages read in one go.

Why darija is written with digits
Darija is written in Arabic letters and, every day, in Latin letters. When Latin keyboards arrived on phones, people needed a way to note the sounds French and English lack. Moroccans picked the digits whose shape recalls the Arabic letter: 3 looks like a flipped ع, 7 like ح, 9 like ق.
The system has no official rule. Everyone writes it a little differently, which is why a darija dictionary must accept several spellings of the same word.
3: the letter ع
A tight throat sound with no equivalent in English. Tighten your throat and say “a”. You hear it in 3afak (please), the most useful word on the list, and in be3d (after).
In the app, the 3afak entry opens when you type 3afak, but also afak: the search knows the 3 can drop. And you can play it, at normal or slow speed, to hear what the 3 does to the voice.
7: the letter ح
A strong breathy h, like fogging up a window. 7lib is milk. sba7 lkhir is good morning. mer7ba is welcome. Don't confuse it with the ordinary softer h, as in l7amdolillah.
9: the letter ق
A k pronounced at the very back of the mouth. 9elb is the heart. In wach 3andek lwra9 (do you have the papers), the final 9 is barely voiced, but it changes the word.
The other letters you meet: kh, gh, ch, and the capitals
kh (خ) sounds like the Spanish jota, a light scrape: khdma, work. gh (غ) is close to the French r: ghedda, tomorrow. ch (ش) reads like “sh”. Capital S, T and D mark heavier consonants, said with the tongue further back.
The app's “Pronunciation and writing” guide, in Profile, goes through each letter with an example and its voice. And the “Read arabizi” track, in Learn, trains you on them in ten minutes.
How to type a word in arabizi to find it
Type it the way you hear it. The dictionary searches in English, arabizi and Arabic letters at once, and it tolerates a typo. If a spelling you use finds nothing, the “Report” button sends it to us: that is a bug on our side, not a mistake on yours.

Questions about this guide
Do 2 and 5 exist in arabizi too?
Yes, more rarely. 2 notes the hamza, a small break in the voice. 5 sometimes replaces kh. The app understands both in the search.
Do I need the Arabic alphabet to read darija?
No. Most texts are in Latin letters with digits. The Arabic script is shown on every entry so you get used to it, with no obligation.