This is the 3293rd most frequent Catalan word.
ull
"Ull" means "eye" in English.
Here, 'ull' refers to the physical organ of sight, specifically the left eye, used for observing.
Vaig veure alguna cosa brillant amb l'ull esquerre.
I saw something shiny with my left eye.
In this sentence, 'ull' metaphorically represents the ability to notice or appreciate details.
Luís sempre té un ull per als detalls.
Luís always has an eye for detail.
Here, 'ull' is used in a literal sense to indicate the action of bringing one's organ of sight near the keyhole for observation.