This is the 1316th most frequent Polish word.
najbliższej
"Najbliższej" means "the nearest" or "the closest" (feminine form, genitive case).
The word 'najbliższej' is used as a locative adjective, modifying 'stacji'. It expresses proximity in physical space.
Najbliższej stacji metra znajduje się 200 metrów stąd.
The nearest metro station is 200 meters from here.
Here, 'najbliższej' serves as a descriptive adjective modifying 'przyszłości', and it denotes closeness in temporal terms.
Przyjadę w najbliższej przyszłości, może za kilka dni.
I will come in the nearest future, maybe in a few days.
In this case, 'najbliższej' functions as a locative adjective modifying 'lekcji', indicating a point in time that is upcoming.