glan as an adjective (as here) generally means "clean". glan can also qualify another adjective, where it means something like "totally", "completely", etc. You can use glan as an interjection on its own to express approval - 'excellent!', 'great!', etc. In informal speech you could use it to mean 'great' etc. in the Duolingo sentence, but without context, spoken intonation, etc. the standard meaning of 'tha sin glan' when written would be 'that is clean'.