This is the 2163rd most frequent Catalan word.
boira
Fog.
The word 'boira' is used here as a noun to refer to the atmospheric phenomenon of fog, obscuring visibility.
La boira cobreix les muntanyes al matí.
The fog covers the mountains in the morning.
In this sentence, 'boira' is again a noun, described with the adjective 'densa', emphasizing the thickness of the fog.
Avui sembla que hi haurà boira densa.
Today it seems there will be dense fog.
Here, 'boira' is used metaphorically to describe an ambiance or feeling that clouds clarity, similar to actual fog.