This is the 4643rd most frequent Russian word.
дневная
"Дневная" means "daytime" or "day's" in English.
The word 'дневная' in this sentence acts as an adjective describing the type of excursion, meaning 'daytime'.
Сегодня у нас дневная экскурсия по городу.
Today, we have a daytime city tour.
Here, 'дневной' (adjective form related to 'дневная') specifies the type of light, referring to 'daylight'.
К дневной свет у меня особенно чувствительные глаза.
My eyes are particularly sensitive to daylight.
In this example, 'дневная' qualifies the care provided, indicating it occurs during the day.