AFAIU

AFAIU means: As far as I understand.. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.

AFAIU means: As far as I understand.. Register: neutral, standard English. A common learner mistake is using the word in a register it does not fit, or assuming a single global meaning; native speakers immediately notice when slang appears in formal contexts, so always check the surrounding register before producing it yourself. A formal-English equivalent (a synonym or descriptive phrase) is usually safer in writing. When in doubt, paraphrase rather than reuse the slang form.