Literally, that means "I don't understand English". Sometimes it is said by Irish speakers to signal their reluctance to speak in English.
I remember a guy who transferred to my (Irish language) school in fourth year, when we would have been aged about sixteen, saying these words to a nun and a teacher, when he was newly arrived and we were both sitting studying in the hall. (The hall had tables in it, for lunch and studying.)
This guy was (and is) a handsome, intelligent, charming guy-- and knew it. (He's a doctor now.) I remember the teacher and nun both laughing rather girlishly when he rolled out this line-- very cute for a guy who had just come from an English speaking school, and might be expected to struggle with Irish.
I've just been watching a video where the speaker says that, on a recent trip to Stockholm, he noticed just how much English was everywhere-- and he even heard one Swede wondering aloud why Swedes should bother learning and speaking Swedish anymore.
That just makes me feel desolate. English was always (by far) my best subject in school, and all my achievements in life have been through the use of the English language. All the literature I love is in English.
But it's also the language of globalization, and hearing something like that just makes me feel like saying: "Ni thuigim Béarla" all the time. (A reaction that passes, but still.)